diff --git a/libkeyringctl/keyring.py b/libkeyringctl/keyring.py
index c4d929b..efbe858 100644
--- a/libkeyringctl/keyring.py
+++ b/libkeyringctl/keyring.py
@@ -354,6 +354,10 @@ def convert_certificate(
             current_packet_fingerprint = None
             current_packet_uid = Uid(simplify_ascii(packet_dump_field(packet, "Value")))
 
+            if current_packet_uid in uids:
+                raise Exception(
+                    f"Duplicate User ID {current_packet_uid} used in packet {uids[current_packet_uid]} and {packet}"
+                )
             uids[current_packet_uid] = packet
         elif packet.name.endswith("UserAttribute"):
             current_packet_mode = "uattr"
diff --git a/tests/test_keyring.py b/tests/test_keyring.py
index 52bbfb3..a48b6d2 100644
--- a/tests/test_keyring.py
+++ b/tests/test_keyring.py
@@ -423,6 +423,22 @@ def test_convert_signature_packet(
             "bar",
             does_not_raise(),
         ),
+        (
+            Path("foo.asc"),
+            [
+                Path("--PublicKey"),
+                Path("--Signature"),
+                Path("--UserID"),
+                Path("--UserID"),
+            ],
+            [
+                "".join(choice("ABCDEF" + digits) for _ in range(40)),
+                "foo <foo@bar.com>",
+                "foo <foo@bar.com>",
+            ],
+            "bar",
+            raises(Exception),
+        ),
         (
             Path("foo.asc"),
             [