diff --git a/components/import-gcsim-dialog.tsx b/components/import-gcsim-dialog.tsx
index 9cd4a91..30d1d22 100644
--- a/components/import-gcsim-dialog.tsx
+++ b/components/import-gcsim-dialog.tsx
@@ -16,6 +16,7 @@ export function ImportGcsimDialog({ catalog, disabled, onImport }: { catalog: Ca
   const [input, setInput] = useState("");
   const [name, setName] = useState("Imported team");
   const preview = useMemo(() => input.trim() ? importGcsimConfig(input, catalog, name) : undefined, [input, catalog, name]);
+  const lineNumbers = useMemo(() => Array.from({ length: input.split("\n").length }, (_, index) => `${index + 1} |`).join("\n"), [input]);
   const error = preview?.error;
   return <Dialog open={open} onOpenChange={setOpen}>
     <DialogTrigger render={<Button variant="outline" className="self-start" disabled={disabled} />}><ImportIcon data-icon="inline-start" />Import from gcsim</DialogTrigger>
@@ -23,7 +24,7 @@ export function ImportGcsimDialog({ catalog, disabled, onImport }: { catalog: Ca
       <DialogHeader><DialogTitle>Import from gcsim</DialogTitle><DialogDescription>Paste character settings, artifact stats, and rotation script. Import creates a new team.</DialogDescription></DialogHeader>
       <FieldGroup>
         <Field><FieldLabel htmlFor="gcsim-import-name">Team name</FieldLabel><Input id="gcsim-import-name" value={name} maxLength={60} onChange={(event) => setName(event.target.value)} /></Field>
-        <Field data-invalid={Boolean(error)}><FieldLabel htmlFor="gcsim-import-input">Gcsim config</FieldLabel><Textarea id="gcsim-import-input" className="min-h-64" value={input} maxLength={100000} onChange={(event) => setInput(event.target.value)} placeholder={'bennett char lvl=90/90 cons=0 talent=9,9,9;\nbennett add weapon="favoniussword" refine=1 lvl=90/90;\nbennett add set="noblesseoblige" count=4;\nbennett add stats hp=4780 atk=311 er=0.518;\n\nactive bennett;\nbennett skill, burst;'} aria-invalid={Boolean(error)} aria-describedby={`gcsim-import-help${error ? " gcsim-import-error" : ""}`} /><FieldDescription id="gcsim-import-help">Exact artifact values are preserved. Loops and action parameters stay as gcsim script. Character and equipment keys must exist in the catalog.</FieldDescription>{error && <FieldError id="gcsim-import-error">{error}</FieldError>}</Field>
+        <Field data-invalid={Boolean(error)}><FieldLabel htmlFor="gcsim-import-input">Gcsim config</FieldLabel><div className="grid grid-cols-[auto_minmax(0,1fr)]"><pre aria-hidden="true" className="select-none py-3 pr-3 text-right font-mono text-sm leading-6 text-muted-foreground">{lineNumbers}</pre><Textarea id="gcsim-import-input" className="min-h-64 font-mono leading-6" wrap="off" value={input} maxLength={100000} onChange={(event) => setInput(event.target.value)} placeholder={'bennett char lvl=90/90 cons=0 talent=9,9,9;\nbennett add weapon="favoniussword" refine=1 lvl=90/90;\nbennett add set="noblesseoblige" count=4;\nbennett add stats hp=4780 atk=311 er=0.518;\n\nactive bennett;\nbennett skill, burst;'} aria-invalid={Boolean(error)} aria-describedby={`gcsim-import-help${error ? " gcsim-import-error" : ""}`} /></div><FieldDescription id="gcsim-import-help">Exact artifact values are preserved. Loops and action parameters stay as gcsim script. Character and equipment keys must exist in the catalog.</FieldDescription>{error && <FieldError id="gcsim-import-error"><span>{error}{preview.errorLine ? ` (line ${preview.errorLine})` : ""}</span>{preview.errorSource && <code className="block whitespace-pre-wrap break-words">{preview.errorLine} | {preview.errorRange ? <>{preview.errorSource.slice(0, preview.errorRange.start)}<u>{preview.errorSource.slice(preview.errorRange.start, preview.errorRange.end)}</u>{preview.errorSource.slice(preview.errorRange.end)}</> : preview.errorSource}</code>}</FieldError>}</Field>
         {preview?.team && <Alert><AlertTitle>Ready to import</AlertTitle><AlertDescription>{preview.team.builds.map((build) => build.characterName).join(", ")} · {preview.team.builds.length} character{preview.team.builds.length === 1 ? "" : "s"} · {preview.team.rotation?.enabled ? "Rotation included" : "No rotation"}</AlertDescription></Alert>}
         {preview?.warnings?.map((warning) => <Alert key={warning}><AlertTitle>Import note</AlertTitle><AlertDescription>{warning}</AlertDescription></Alert>)}
       </FieldGroup>
diff --git a/lib/translator/gcsim-text.ts b/lib/translator/gcsim-text.ts
index b2bce7b..b99f5c5 100644
--- a/lib/translator/gcsim-text.ts
+++ b/lib/translator/gcsim-text.ts
@@ -1,14 +1,38 @@
+export type GcsimStatement = { text: string; line: number; source: string };
+
+export class GcsimTextError extends Error {
+  constructor(message: string, public line: number, public source: string) {
+    super(message);
+  }
+}
+
+function lineDetails(input: string, position: number) {
+  const line = input.slice(0, position).split("\n").length;
+  return { line, source: input.split(/\r?\n/)[line - 1]?.trim() ?? "" };
+}
+
 // Split only at top level: loops, conditionals, functions and action parameters
 // must stay intact. Comments are removed without touching quoted strings.
-export function splitGcsimStatements(input: string): string[] {
-  const statements: string[] = [];
+export function splitGcsimStatementsWithLocations(input: string): GcsimStatement[] {
+  const originalInput = input;
+  const statements: GcsimStatement[] = [];
   const stack: string[] = [];
   // Replacing comments up front also lets a comment appear before an else.
-  input = input.replace(/"(?:\\.|[^"\\])*"|'(?:\\.|[^'\\])*'|#[^\n]*|\/\/[^\n]*/g, (match) => match.startsWith("#") || match.startsWith("//") ? " " : match);
+  input = input.replace(/"(?:\\.|[^"\\])*"|'(?:\\.|[^'\\])*'|#[^\n]*|\/\/[^\n]*/g, (match) => match.startsWith("#") || match.startsWith("//") ? " ".repeat(match.length) : match);
   let text = "";
+  let start = 0;
   let quote = "";
   let escaped = false;
-  const flush = () => { if (text.trim()) statements.push(text.trim()); text = ""; };
+  const flush = (end: number) => {
+    const leadingWhitespace = text.search(/\S/);
+    if (leadingWhitespace >= 0) {
+      const position = start + leadingWhitespace;
+      const { line, source } = lineDetails(originalInput, position);
+      statements.push({ text: text.trim(), line, source });
+    }
+    text = "";
+    start = end;
+  };
   for (let i = 0; i < input.length; i++) {
     const char = input[i];
     if (quote) {
@@ -21,18 +45,30 @@ export function splitGcsimStatements(input: string): string[] {
     if (char === '"' || char === "'") { quote = char; text += char; continue; }
     if ("([{".includes(char)) {
       stack.push(char);
-      if (stack.length > 64) throw new Error("Config is nested too deeply.");
+      if (stack.length > 64) {
+        const details = lineDetails(originalInput, i);
+        throw new GcsimTextError("Config is nested too deeply.", details.line, details.source);
+      }
     } else if (")]}".includes(char)) {
       const expected: Record<string, string> = { ")": "(", "]": "[", "}": "{" };
-      if (stack.pop() !== expected[char]) throw new Error(`Unmatched '${char}' at position ${i + 1}.`);
+      if (stack.pop() !== expected[char]) {
+        const details = lineDetails(originalInput, i);
+        throw new GcsimTextError(`Unmatched '${char}' at position ${i + 1}.`, details.line, details.source);
+      }
     }
     text += char;
-    if (stack.length === 0 && char === ";") flush();
+    if (stack.length === 0 && char === ";") flush(i + 1);
     // Keep an else branch attached to its if statement.
-    if (stack.length === 0 && char === "}" && !/^\s*else\b/.test(input.slice(i + 1))) flush();
+    if (stack.length === 0 && char === "}" && !/^\s*else\b/.test(input.slice(i + 1))) flush(i + 1);
   }
-  if (quote) throw new Error("Unclosed quoted string.");
-  if (stack.length) throw new Error("Unclosed bracket or parenthesis.");
-  if (text.trim()) throw new Error("Missing ';' at the end of a statement.");
+  const trailingPosition = start + Math.max(0, text.search(/\S/));
+  const trailing = lineDetails(originalInput, trailingPosition);
+  if (quote) throw new GcsimTextError("Unclosed quoted string.", trailing.line, trailing.source);
+  if (stack.length) throw new GcsimTextError("Unclosed bracket or parenthesis.", trailing.line, trailing.source);
+  if (text.trim()) throw new GcsimTextError("Missing ';' at the end of a statement.", trailing.line, trailing.source);
   return statements;
 }
+
+export function splitGcsimStatements(input: string): string[] {
+  return splitGcsimStatementsWithLocations(input).map((statement) => statement.text);
+}
diff --git a/lib/translator/import.test.ts b/lib/translator/import.test.ts
index 274196d..eb61778 100644
--- a/lib/translator/import.test.ts
+++ b/lib/translator/import.test.ts
@@ -214,6 +214,18 @@ describe("gcsim import", () => {
     expect(result.team).toBeUndefined();
     expect(result.error).toContain(expected);
   });
+  it("reports the source line for duplicate stats", () => {
+    const input = config.replace("hp=717 atk%=0.1234567 cr=0.128 cd=0.265", "hp=717 def%=0.1 def%=0.2");
+    const result = importGcsimConfig(input, catalog);
+    const expectedLine = input.split("\n").findIndex((line) => line.includes("def%=0.1")) + 1;
+    expect(result).toMatchObject({
+      error: "Duplicate build setting 'def%'.",
+      errorLine: expectedLine,
+    });
+    expect(result.errorSource).toContain("def%=0.1 def%=0.2");
+    expect(result.errorSource?.slice(result.errorRange?.start, result.errorRange?.end)).toBe("def%");
+    expect(result.errorRange?.start).toBe(result.errorSource?.lastIndexOf("def%"));
+  });
   it("limits characters and input size", () => {
     const declarations = catalog.characters.map((char) => `${char.key} char lvl=90/90 cons=0 talent=9,9,9;`).join("\n");
     expect(importGcsimConfig(declarations, catalog).error).toContain("four");
diff --git a/lib/translator/import.ts b/lib/translator/import.ts
index f4fb6d3..bbe4839 100644
--- a/lib/translator/import.ts
+++ b/lib/translator/import.ts
@@ -1,11 +1,22 @@
 import { createBuild } from "./build";
-import { splitGcsimStatements } from "./gcsim-text";
+import { GcsimTextError, splitGcsimStatementsWithLocations, type GcsimStatement } from "./gcsim-text";
 import { translateRotation } from "./rotation";
 import { isRotationDefaultStatement } from "./rotation-defaults";
 import { canonicalArtifactKey, canonicalCharacterKey, canonicalWeaponKey } from "./keys";
 import { importedStatKeys, translationSchema, type Build, type Catalog, type CatalogItem, type Team } from "./types";
 
-export type ImportResult = { team: Team; warnings: string[]; error?: never } | { error: string; team?: never; warnings?: never };
+type ErrorRange = { start: number; end: number };
+export type ImportResult = { team: Team; warnings: string[]; error?: never; errorLine?: never; errorSource?: never; errorRange?: never } | { error: string; errorLine?: number; errorSource?: string; errorRange?: ErrorRange; team?: never; warnings?: never };
+
+function findErrorRange(message: string, source?: string): ErrorRange | undefined {
+  if (!source) return;
+  const token = /'([^']+)'/.exec(message)?.[1];
+  if (!token) return;
+  const normalizedSource = source.toLowerCase();
+  const normalizedToken = token.toLowerCase();
+  const start = message.startsWith("Duplicate ") ? normalizedSource.lastIndexOf(normalizedToken) : normalizedSource.indexOf(normalizedToken);
+  return start >= 0 ? { start, end: start + token.length } : undefined;
+}
 function findItem(items: CatalogItem[], key: string, kind: string): CatalogItem {
   const canonicalize = kind === "character" ? canonicalCharacterKey : kind === "weapon" ? canonicalWeaponKey : canonicalArtifactKey;
   const canonical = canonicalize(key);
@@ -64,14 +75,17 @@ function addStatValues(left: number, right: number): number {
 export function importGcsimConfig(input: string, catalog: Catalog, name = "Imported team"): ImportResult {
   if (!input.trim()) return { error: "Paste a gcsim config to import." };
   if (input.length > 100000) return { error: "Config exceeds 100,000 characters." };
+  let currentStatement: GcsimStatement | undefined;
   try {
-    const statements = splitGcsimStatements(input);
+    const statements = splitGcsimStatementsWithLocations(input);
     const builds = new Map<string, Build>();
     const ids = new Set<string>();
     const weaponSeen = new Set<string>();
     const warnings = new Set<string>();
     const script: string[] = [];
-    for (const statement of statements) {
+    for (const locatedStatement of statements) {
+      currentStatement = locatedStatement;
+      const statement = locatedStatement.text;
       const match = /^([a-z]+)\s+char\s+([\s\S]*);$/i.exec(statement);
       if (!match) continue;
       const key = match[1].toLowerCase();
@@ -86,6 +100,7 @@ export function importGcsimConfig(input: string, catalog: Catalog, name = "Impor
       if (!/^\d+,\d+,\d+$/.test(talents)) throw new Error(`Invalid talent levels for '${key}'.`);
       builds.set(key, { ...build, characterKey: key, level: lvl, maxLevel: cap, constellation: Number(required(values, "cons")), talents: talents.split(",").map(Number) as Build["talents"], importedStats: {}, importedSets: [], importedParams: params ? { character: params } : undefined });
     }
+    currentStatement = undefined;
     if (!builds.size) throw new Error("No character declarations found. Include lines such as 'bennett char lvl=90/90 cons=0 talent=9,9,9;'.");
     const resolveBuild = (key: string): Build => {
       const item = findItem(catalog.characters, key, "character");
@@ -93,7 +108,9 @@ export function importGcsimConfig(input: string, catalog: Catalog, name = "Impor
       if (!build) throw new Error(`Character '${key}' has settings but no character declaration.`);
       return build;
     };
-    for (const statement of statements) {
+    for (const locatedStatement of statements) {
+      currentStatement = locatedStatement;
+      const statement = locatedStatement.text;
       if (/^[a-z]+\s+char\b/i.test(statement)) continue;
       const setting = /^([a-z]+)\s+add\s+([\s\S]*);$/i.exec(statement);
       if (setting) {
@@ -130,6 +147,7 @@ export function importGcsimConfig(input: string, catalog: Catalog, name = "Impor
       if (isRotationDefaultStatement(statement)) continue;
       script.push(statement);
     }
+    currentStatement = undefined;
     for (const build of builds.values()) {
       if (!weaponSeen.has(build.characterId)) throw new Error(`Missing weapon settings for '${build.characterName}'.`);
       if (build.importedSets!.reduce((sum, set) => sum + set.count, 0) > 5) throw new Error(`Artifact count exceeds five for '${build.characterName}'.`);
@@ -146,6 +164,8 @@ export function importGcsimConfig(input: string, catalog: Catalog, name = "Impor
     if (!validated.success) throw new Error(validated.error.issues.map((issue) => `${issue.path.join(".")}: ${issue.message}`).join(" "));
     return { team: validated.data.teams[0], warnings: [...warnings] };
   } catch (error) {
-    return { error: (error as Error).message };
+    if (error instanceof GcsimTextError) return { error: error.message, errorLine: error.line, errorSource: error.source, errorRange: findErrorRange(error.message, error.source) };
+    const message = (error as Error).message;
+    return { error: message, errorLine: currentStatement?.line, errorSource: currentStatement?.source, errorRange: findErrorRange(message, currentStatement?.source) };
   }
 }