diff --git a/src/components/CommitAuthors.svelte b/src/components/CommitAuthors.svelte
index 9147d3ba7..ce36ea404 100644
--- a/src/components/CommitAuthors.svelte
+++ b/src/components/CommitAuthors.svelte
@@ -19,31 +19,7 @@
   // Which identity was last copied, so the rows keep independent feedback.
   let copied: string | undefined = $state();
 
-  // The identity as it appears in a commit trailer, so that a copied result can
-  // be pasted straight into `Co-authored-by:` and the like.
-  function identity(who: GitIdentity): string {
-    return who.name ? `${who.name} <${who.email}>` : who.email;
-  }
-
-  // Co-authors are not part of the commit's git identity fields; they are a
-  // convention carried in the message trailers. The same person can be
-  // trailered more than once, and squashing gathers several messages' trailers
-  // into one, so only the first mention of each is kept.
-  const coAuthors = $derived.by(() => {
-    const credited = new Set([
-      utils.identityKey(commit.author),
-      utils.identityKey(commit.committer),
-    ]);
-
-    return utils.coAuthors(commit.message).filter((who, index, all) => {
-      const key = utils.identityKey(who);
-
-      return (
-        !credited.has(key) &&
-        all.findIndex(other => utils.identityKey(other) === key) === index
-      );
-    });
-  });
+  const coAuthors = $derived(utils.creditedCoAuthors(commit));
 
   const authorIsCommitter = $derived(
     utils.identityKey(commit.author) === utils.identityKey(commit.committer),
@@ -66,7 +42,7 @@
   }, 1000);
 
   async function copyIdentity(who: GitIdentity) {
-    await writeToClipboard(identity(who));
+    await writeToClipboard(utils.formatGitIdentity(who));
     copied = utils.identityKey(who);
     restoreIcon();
   }
@@ -223,7 +199,7 @@
               type="button"
               class="email-row"
               class:nameless={!who.name}
-              title={`Copy "${identity(who)}"`}
+              title={`Copy "${utils.formatGitIdentity(who)}"`}
               onclick={event => {
                 event.stopPropagation();
                 void copyIdentity(who);
diff --git a/src/lib/utils.ts b/src/lib/utils.ts
index fc01caebb..9eb10e4eb 100644
--- a/src/lib/utils.ts
+++ b/src/lib/utils.ts
@@ -445,3 +445,30 @@ export function coAuthors(message: string): GitIdentity[] {
 
   return authors;
 }
+
+// The identity as it appears in a commit trailer, so that a copied result can
+// be pasted straight into `Co-authored-by:` and the like.
+export function formatGitIdentity(who: GitIdentity): string {
+  return who.name ? `${who.name} <${who.email}>` : who.email;
+}
+
+// Co-authors are not part of the commit's git identity fields; they are a
+// convention carried in the message trailers. The same person can be
+// trailered more than once, and squashing gathers several messages' trailers
+// into one, so only the first mention of each is kept.
+export function creditedCoAuthors(commit: {
+  author: GitIdentity;
+  committer: GitIdentity;
+  message: string;
+}): GitIdentity[] {
+  const credited = new Set([
+    identityKey(commit.author),
+    identityKey(commit.committer),
+  ]);
+  return coAuthors(commit.message).filter(who => {
+    const key = identityKey(who);
+    if (credited.has(key)) return false;
+    credited.add(key);
+    return true;
+  });
+}
diff --git a/tests/unit/utils.test.ts b/tests/unit/utils.test.ts
index 017bb595f..4331eaad6 100644
--- a/tests/unit/utils.test.ts
+++ b/tests/unit/utils.test.ts
@@ -4,8 +4,10 @@ import { describe, expect, test, vi } from "vitest";
 
 import {
   coAuthors,
+  creditedCoAuthors,
   explorerHost,
   explorerUrl,
+  formatGitIdentity,
   formatRepositoryId,
   formatTimestamp,
   identityKey,
@@ -301,3 +303,29 @@ describe("verdicts", () => {
     },
   );
 });
+
+describe("commit credits", () => {
+  const alice = { name: "Alice", email: "alice@example.com" };
+  const bob = { name: "Bob", email: "bob@example.com" };
+  const carol = { name: "Carol", email: "carol@example.com" };
+
+  test("formatGitIdentity writes a trailer identity", () => {
+    expect(formatGitIdentity(alice)).toBe("Alice <alice@example.com>");
+    expect(formatGitIdentity({ name: "", email: "a@b.c" })).toBe("a@b.c");
+  });
+
+  test("creditedCoAuthors skips the author, the committer and repeats", () => {
+    const message = [
+      "Squashed",
+      "",
+      "Co-authored-by: Alice <ALICE@example.com>",
+      "Co-authored-by: Bob <bob@example.com>",
+      "Co-authored-by: Carol <carol@example.com>",
+      "Co-authored-by: Carol C. <Carol@example.com>",
+    ].join("\n");
+
+    expect(
+      creditedCoAuthors({ author: alice, committer: bob, message }),
+    ).toEqual([carol]);
+  });
+});