The reason why the card number is showing duplicate for Access Card when enrolling to Access Control Systems, is that the card reader might be reading only the first 6 to 8 numbers of the card.

Here is the example.

Card numbers are;

1234500001

1234500002

1234500003

As you can see if their card reader is only reading the first 6 to 8 numbers of the card, therefore it show duplicates.