Monitoring - We identified an issue that affected users that tried to re-link the same card or a card that already existed in the Fidel platform. The issue was live from Friday 5th 4:00pm to Tuesday 9th 6am. It was investigated since Monday 8th 1pm.
The issue has been fixed and we're monitoring to guarantee it is resolved. Please contact support@fidel.uk if you have any additional questions.

More information:
For Mastercard cardholders, the issue prevented an existing card to be linked again by showing the message "Invalid Card". For Visa cardholders, the issue caused the duplication of the existing Visa card.
Apr 9, 13:18 BST