feat(checks): add duplicate-name rule

Warn when two or more skills declare the same frontmatter name. At runtime
the selection is ambiguous and one skill silently shadows the other, so
this is a real bug with no visible signal in model output.

The check follows the cross-skill pattern of description-collision:
collect all validated skills, group by lowercased name, then emit one
diagnostic per skill in any group with two or more members. Each
diagnostic names the conflicting file so the author knows exactly which
skill to rename.

Four new tests cover: two-way duplicate, three-way duplicate, no duplicate
(no diagnostic), and the diagnostic message content.

Author: protosphinx

README.md

@@ -98,6 +98,7 @@ Exit codes:
 | `name-drift` | warn | Frontmatter `name:` doesn't match the filename or directory |
 | `description-collision` | warn | Two skills' descriptions have Jaccard ≥ 0.6 |
 | `tools-overloaded` | warn | `tools:` lists 10 or more entries; narrow the list to what this skill actually needs |
+| `duplicate-name` | warn | Two or more skills share the same `name:` value; resolution is ambiguous |
 | `parse` | error | The file doesn't have valid frontmatter / YAML |
 
 The MCP and built-in tool checks read `~/.claude/settings.json` and

src/checks.ts

@@ -36,6 +36,7 @@ export function runChecks(
   }
 
   diagnostics.push(...checkCollisions(validated));
+  diagnostics.push(...checkDuplicateNames(validated));
   return diagnostics;
 }
 
@@ -206,3 +207,30 @@ function checkCollisions(skills: ValidatedSkill[]): Diagnostic[] {
   }
   return out;
 }
+
+function checkDuplicateNames(skills: ValidatedSkill[]): Diagnostic[] {
+  const byName = new Map<string, ValidatedSkill[]>();
+  for (const s of skills) {
+    const key = s.name.toLowerCase();
+    const group = byName.get(key) ?? [];
+    group.push(s);
+    byName.set(key, group);
+  }
+  const out: Diagnostic[] = [];
+  for (const group of byName.values()) {
+    if (group.length < 2) continue;
+    for (const s of group) {
+      const others = group
+        .filter((g) => g.file !== s.file)
+        .map((g) => `'${g.file}'`)
+        .join(", ");
+      out.push({
+        severity: "warn",
+        rule: "duplicate-name",
+        message: `skill name '${s.name}' is also declared in ${others}`,
+        file: s.file,
+      });
+    }
+  }
+  return out;
+}

test/checks.test.ts

@@ -234,4 +234,34 @@ describe("runChecks", () => {
     const ds = runChecks([s], config);
     expect(ds.find((d) => d.rule === "empty-body")).toBeUndefined();
   });
+
+  it("flags duplicate skill names", () => {
+    const a = mkSkill("/test/a/deploy.md", { name: "deploy", description: "deploy the app" });
+    const b = mkSkill("/test/b/deploy.md", { name: "deploy", description: "deploy to staging" });
+    const ds = runChecks([a, b], config);
+    expect(ds.filter((d) => d.rule === "duplicate-name").length).toBe(2);
+  });
+
+  it("does not flag duplicate-name for unique names", () => {
+    const a = mkSkill("/test/a/deploy.md", { name: "deploy", description: "deploy the app" });
+    const b = mkSkill("/test/b/release.md", { name: "release", description: "cut a release" });
+    const ds = runChecks([a, b], config);
+    expect(ds.find((d) => d.rule === "duplicate-name")).toBeUndefined();
+  });
+
+  it("flags all skills in a three-way duplicate-name group", () => {
+    const a = mkSkill("/test/a/foo.md", { name: "foo", description: "do foo one way" });
+    const b = mkSkill("/test/b/foo.md", { name: "foo", description: "do foo another way" });
+    const c = mkSkill("/test/c/foo.md", { name: "foo", description: "do foo a third way" });
+    const ds = runChecks([a, b, c], config);
+    expect(ds.filter((d) => d.rule === "duplicate-name").length).toBe(3);
+  });
+
+  it("duplicate-name message names the conflicting file", () => {
+    const a = mkSkill("/test/a/deploy.md", { name: "deploy", description: "deploy the app" });
+    const b = mkSkill("/test/b/deploy.md", { name: "deploy", description: "deploy to staging" });
+    const ds = runChecks([a, b], config);
+    const diagA = ds.find((d) => d.rule === "duplicate-name" && d.file === "/test/a/deploy.md");
+    expect(diagA?.message).toContain("/test/b/deploy.md");
+  });
 });