fix(hooks): store a hook delivered twice only once

A doubled hook registration (plugin + leftover checkout install) POSTs every
event twice with a byte-identical payload, so the timeline showed each row
twice and token/alert/lane side effects ran twice. POST /api/hooks/event now
skips a payload identical to the session's LATEST event from the last 10s
(findRecentDuplicateEvent). Latest-only keeps two legitimately identical
Stop events of consecutive turns, which always have other events between.

server/__tests__/api.test.js

@@ -617,6 +617,37 @@ describe("Hook Event Processing", () => {
     assert.equal(res.body.error.code, "MISSING_SESSION");
   });
 
+  it("stores a hook delivered twice only once (double registration)", async () => {
+    const hook = {
+      hook_type: "PreToolUse",
+      data: {
+        session_id: "hook-dup-1",
+        tool_name: "Bash",
+        tool_use_id: "toolu_dup_1",
+        tool_input: { command: "ls", description: "List files" },
+      },
+    };
+    const first = await post("/api/hooks/event", hook);
+    const second = await post("/api/hooks/event", hook);
+    assert.equal(first.status, 200);
+    assert.equal(second.status, 200);
+    assert.equal(second.body.duplicate, true);
+    const { n } = db
+      .prepare("SELECT COUNT(*) AS n FROM events WHERE session_id = 'hook-dup-1'")
+      .get();
+    assert.equal(n, 1);
+
+    // A DIFFERENT payload in the same session still lands.
+    await post("/api/hooks/event", {
+      ...hook,
+      data: { ...hook.data, tool_use_id: "toolu_dup_2" },
+    });
+    const after = db
+      .prepare("SELECT COUNT(*) AS n FROM events WHERE session_id = 'hook-dup-1'")
+      .get();
+    assert.equal(after.n, 2);
+  });
+
   it("should auto-create session and main agent on first PreToolUse", async () => {
     const res = await post("/api/hooks/event", {
       hook_type: "PreToolUse",