When receiving singleton credential, remove all other instances

Before 42a3acc2, more than one instance of the singleton credential type may have been stored.

irmaclient/client.go

@@ -224,8 +224,10 @@ func (client *Client) addCredential(cred *credential, storeAttributes bool) (err
 	}
 
 	// If this is a singleton credential type, ensure we have at most one by removing any previous instance
-	if !id.Empty() && cred.CredentialType().IsSingleton && len(client.attrs(id)) > 0 {
-		client.remove(id, 0, false) // Index is 0, because if we're here we have exactly one
+	if !id.Empty() && cred.CredentialType().IsSingleton {
+		for len(client.attrs(id)) != 0 {
+			client.remove(id, 0, false)
+		}
 	}
 
 	// Append the new cred to our attributes and credentials