DEVX-125: feat: detect double-prefix in Vikunja task title during pre-merge validation

tests/unit/test_check_auto_merge_ready.py

@@ -292,3 +292,18 @@ class TestCli:
             )
         assert result.exit_code != 0
         assert "does not match Vikunja" in result.output
+
+    def test_fails_with_double_prefix_in_vikunja_title(self) -> None:
+        """Vikunja title with task ID prefix causes double-prefix in PR title."""
+        runner = CliRunner()
+        with (
+            patch.dict("os.environ", {"DEVX_TASK_PREFIX": "DEVX", "VIKUNJA_TOKEN": "tok"}, clear=True),
+            patch("devx.ci.check_auto_merge_ready.is_branch_behind_master", return_value=False),
+            patch("devx.ci.check_auto_merge_ready.get_vikunja_title_optional", return_value="DEVX-1: Fix foo"),
+        ):
+            result = runner.invoke(
+                cli,
+                ["--branch", "DEVX-1-fix-foo", "--pr-title", "DEVX-1: Fix foo"],
+            )
+        assert result.exit_code != 0
+        assert "should NOT include" in result.output