diff --git a/signature/internal/sigstore_payload.go b/signature/internal/sigstore_payload.go
index 12dd6408ee..5ee1156793 100644
--- a/signature/internal/sigstore_payload.go
+++ b/signature/internal/sigstore_payload.go
@@ -120,7 +120,7 @@ func (s *UntrustedSigstorePayload) strictUnmarshalJSON(data []byte) error {
 	if gotTimestamp {
 		intTimestamp := int64(timestamp)
 		if float64(intTimestamp) != timestamp {
-			return NewInvalidSignatureError("Field optional.timestamp is not is not an integer")
+			return NewInvalidSignatureError("Field optional.timestamp is not an integer")
 		}
 		s.untrustedTimestamp = &intTimestamp
 	}
diff --git a/signature/simple.go b/signature/simple.go
index 634c920e08..94a8465930 100644
--- a/signature/simple.go
+++ b/signature/simple.go
@@ -143,7 +143,7 @@ func (s *untrustedSignature) strictUnmarshalJSON(data []byte) error {
 	if gotTimestamp {
 		intTimestamp := int64(timestamp)
 		if float64(intTimestamp) != timestamp {
-			return internal.NewInvalidSignatureError("Field optional.timestamp is not is not an integer")
+			return internal.NewInvalidSignatureError("Field optional.timestamp is not an integer")
 		}
 		s.untrustedTimestamp = &intTimestamp
 	}