diff --git a/apps/client/src/services/i18n.spec.ts b/apps/client/src/services/i18n.spec.ts
index e408816436..3216c6cf0c 100644
--- a/apps/client/src/services/i18n.spec.ts
+++ b/apps/client/src/services/i18n.spec.ts
@@ -1,10 +1,10 @@
-import { LOCALES } from "@triliumnext/commons";
+import { findDuplicateJsonKeys, LOCALES } from "@triliumnext/commons";
 import { readFileSync } from "fs";
 import { join } from "path";
 import { describe, expect, it } from "vitest";
 
 describe("i18n", () => {
-    it("translations are valid JSON", () => {
+    it("translations are valid JSON with no duplicate keys", () => {
         for (const locale of LOCALES) {
             if (locale.contentOnly || locale.id === "en_rtl") {
                 continue;
@@ -14,6 +14,13 @@ describe("i18n", () => {
             const translationFile = readFileSync(translationPath, { encoding: "utf-8" });
             expect(() => JSON.parse(translationFile), `JSON error while parsing locale '${locale.id}' at "${translationPath}"`)
                 .not.toThrow();
+
+            const duplicates = findDuplicateJsonKeys(translationFile);
+            expect(
+                duplicates,
+                `Duplicate keys in locale '${locale.id}' at "${translationPath}":\n${ 
+                    duplicates.map((d) => `  - "${d.key}" (line ${d.line})`).join("\n")}`
+            ).toEqual([]);
         }
     });
 });
diff --git a/apps/server/src/services/i18n.spec.ts b/apps/server/src/services/i18n.spec.ts
index e9be7efc81..fba64cdb7b 100644
--- a/apps/server/src/services/i18n.spec.ts
+++ b/apps/server/src/services/i18n.spec.ts
@@ -1,4 +1,4 @@
-import { LOCALES } from "@triliumnext/commons";
+import { findDuplicateJsonKeys, LOCALES } from "@triliumnext/commons";
 import { readFileSync } from "fs";
 import { join } from "path";
 // eslint-disable-next-line @typescript-eslint/no-require-imports
@@ -6,7 +6,7 @@ const { languages } = require("tesseract.js");
 import { describe, expect, it } from "vitest";
 
 describe("i18n", () => {
-    it("translations are valid JSON", () => {
+    it("translations are valid JSON with no duplicate keys", () => {
         for (const locale of LOCALES) {
             if (locale.contentOnly || locale.id === "en_rtl") {
                 continue;
@@ -16,6 +16,13 @@ describe("i18n", () => {
             const translationFile = readFileSync(translationPath, { encoding: "utf-8" });
             expect(() => JSON.parse(translationFile), `JSON error while parsing locale '${locale.id}' at "${translationPath}"`)
                 .not.toThrow();
+
+            const duplicates = findDuplicateJsonKeys(translationFile);
+            expect(
+                duplicates,
+                `Duplicate keys in locale '${locale.id}' at "${translationPath}":\n` +
+                    duplicates.map((d) => `  - "${d.key}" (line ${d.line})`).join("\n")
+            ).toEqual([]);
         }
     });
 
diff --git a/packages/commons/src/lib/test-utils.spec.ts b/packages/commons/src/lib/test-utils.spec.ts
index 4d02fc90d1..90388f5814 100644
--- a/packages/commons/src/lib/test-utils.spec.ts
+++ b/packages/commons/src/lib/test-utils.spec.ts
@@ -1,5 +1,5 @@
 import { describe, it, expect } from "vitest";
-import { trimIndentation } from "./test-utils.js";
+import { findDuplicateJsonKeys, trimIndentation } from "./test-utils.js";
 
 describe("Utils", () => {
     it("trims indentation", () => {
@@ -11,4 +11,27 @@ Hello
     world
 123`);
     });
+
+    describe("findDuplicateJsonKeys", () => {
+        it("returns empty for valid JSON without duplicates", () => {
+            expect(findDuplicateJsonKeys(`{"a": 1, "b": {"c": 2}}`)).toEqual([]);
+        });
+
+        it("detects duplicates at the top level and reports line numbers", () => {
+            const text = `{\n  "a": 1,\n  "b": 2,\n  "a": 3\n}`;
+            expect(findDuplicateJsonKeys(text)).toEqual([{ key: "a", line: 4 }]);
+        });
+
+        it("scopes keys per object — same name at different levels is not a duplicate", () => {
+            expect(findDuplicateJsonKeys(`{"a": {"x": 1}, "b": {"x": 2}}`)).toEqual([]);
+        });
+
+        it("does not treat string values containing a colon as keys", () => {
+            expect(findDuplicateJsonKeys(`{"a": "b:c", "d": "a:e"}`)).toEqual([]);
+        });
+
+        it("does not treat strings inside arrays as keys", () => {
+            expect(findDuplicateJsonKeys(`{"items": ["a", "a", "b"]}`)).toEqual([]);
+        });
+    });
 });
diff --git a/packages/commons/src/lib/test-utils.ts b/packages/commons/src/lib/test-utils.ts
index 86ebfb0d60..a570d00440 100644
--- a/packages/commons/src/lib/test-utils.ts
+++ b/packages/commons/src/lib/test-utils.ts
@@ -62,3 +62,64 @@ export function flushPromises() {
 export function sleepFor(duration: number) {
     return new Promise(resolve => setTimeout(resolve, duration));
 }
+
+/**
+ * Scans raw JSON text for keys that are duplicated within the same object.
+ *
+ * `JSON.parse` silently collapses duplicate keys (the last one wins), which makes
+ * it impossible to detect them from the parsed value. This scanner walks the raw
+ * text, pushing/popping a scope for each `{`/`}`, and identifies a string as a
+ * key when the next non-whitespace char is `:`.
+ *
+ * Intended for validating hand-maintained JSON files (e.g. translation files)
+ * at test level.
+ */
+export function findDuplicateJsonKeys(text: string): Array<{ key: string; line: number }> {
+    const duplicates: Array<{ key: string; line: number }> = [];
+    const stack: Set<string>[] = [];
+    let line = 1;
+    let i = 0;
+
+    while (i < text.length) {
+        const c = text[i];
+        if (c === "\n") {
+            line++;
+            i++;
+        } else if (c === "{") {
+            stack.push(new Set());
+            i++;
+        } else if (c === "}") {
+            stack.pop();
+            i++;
+        } else if (c === '"') {
+            const start = i;
+            const startLine = line;
+            i++;
+            while (i < text.length && text[i] !== '"') {
+                if (text[i] === "\\") {
+                    i += 2;
+                } else {
+                    if (text[i] === "\n") line++;
+                    i++;
+                }
+            }
+            i++;
+            // A string is a key iff the next non-whitespace char is ':'.
+            let j = i;
+            while (j < text.length && /\s/.test(text[j])) j++;
+            if (text[j] === ":" && stack.length > 0) {
+                const key = JSON.parse(text.substring(start, i)) as string;
+                const frame = stack[stack.length - 1];
+                if (frame.has(key)) {
+                    duplicates.push({ key, line: startLine });
+                } else {
+                    frame.add(key);
+                }
+            }
+        } else {
+            i++;
+        }
+    }
+
+    return duplicates;
+}