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.