diff --git a/package.json b/package.json
index 5f14b24..70ad135 100644
--- a/package.json
+++ b/package.json
@@ -46,6 +46,6 @@
   "dependencies": {
     "@openally/result": "2.0.0",
     "lru-cache": "11.2.6",
-    "undici": "7.24.4"
+    "undici": "8.7.0"
   }
 }
diff --git a/src/utils.ts b/src/utils.ts
index b465a76..83865e4 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -34,6 +34,27 @@ export function getEncodingCharset(
   return charset in kCharsetConversionTable ? kCharsetConversionTable[charset] : "utf-8";
 }
 
+/**
+ * @description Set a header case-insensitively, removing any existing entry that
+ * differs only by casing. HTTP header names are case-insensitive and HTTP/2 forbids
+ * singular headers (e.g. `user-agent`) from carrying multiple values, so we must not
+ * end up with both `user-agent` and `User-Agent` in the same object.
+ */
+function setHeader(
+  headers: IncomingHttpHeaders,
+  name: string,
+  value: IncomingHttpHeaders[string]
+): void {
+  const lowerName = name.toLowerCase();
+  for (const key of Object.keys(headers)) {
+    if (key.toLowerCase() === lowerName) {
+      delete headers[key];
+    }
+  }
+
+  headers[name] = value;
+}
+
 /**
  * @description Create a default plain Object headers that will contains a Set of default values like:
  * - User-agent
@@ -42,10 +63,14 @@ export function getEncodingCharset(
 export function createHeaders(
   options: Partial<Pick<RequestOptions, "headers" | "authorization">>
 ): IncomingHttpHeaders {
-  const headers = Object.assign({ ...DEFAULT_HEADER }, options.headers ?? {});
+  const headers: IncomingHttpHeaders = { ...DEFAULT_HEADER };
+
+  for (const [name, value] of Object.entries(options.headers ?? {})) {
+    setHeader(headers, name, value);
+  }
 
   if (options.authorization) {
-    headers.Authorization = createAuthorizationHeader(options.authorization);
+    setHeader(headers, "Authorization", createAuthorizationHeader(options.authorization));
   }
 
   return headers;
diff --git a/test/utils.test.ts b/test/utils.test.ts
index 56c0c99..5f788d4 100644
--- a/test/utils.test.ts
+++ b/test/utils.test.ts
@@ -67,6 +67,17 @@ describe("createHeaders", () => {
     assert.deepStrictEqual(result, { "user-agent": "myUserAgent" });
   });
 
+  it("should overwrite the default 'user-agent' header regardless of casing (no duplicate)", () => {
+    // A caller-provided header must replace the default one even when the casing
+    // differs, otherwise both 'user-agent' and 'User-Agent' end up in the object
+    // and HTTP/2 rejects the request with ERR_HTTP2_HEADER_SINGLE_VALUE.
+    const result = Utils.createHeaders({
+      headers: { "User-Agent": "myUserAgent" }
+    });
+
+    assert.deepStrictEqual(result, { "User-Agent": "myUserAgent" });
+  });
+
   it("should add authorization header (and override original property)", () => {
     const result = Utils.createHeaders({
       headers: {