[GEMINI] Add test case for unique ROT13 metadata decoding

tests/test_decoding.py

@@ -118,3 +118,18 @@ def test_try_decode_metadata_unanchored():
     res = try_decode_metadata("steghide:cEF6endvcmQ=")
     assert res.get("base64") == "pAzzword"
 # }}}
+
+# {{{ test_try_decode_metadata_rot13_unique_only
+def test_try_decode_metadata_rot13_unique_only():
+    """
+    Verifies that rot13 only decodes unique flags and ignores flags already visible.
+    """
+    # Case 1: Flag is encoded and not present in plaintext in the original string.
+    res = try_decode_metadata("synt{ebg13_havdhr}")
+    assert res.get("rot13") == "flag{rot13_unique}"
+
+    # Case 2: Flag is already present in plaintext in the original string.
+    res_duplicate = try_decode_metadata("flag{rot13_unique} synt{ebg13_havdhr}")
+    assert "rot13" not in res_duplicate
+# }}}
+