Can you describe the tables and relationships you have in place for "issuing cards"? That will help us to help you with a way to capture this user error.
There is one table called fuel cards. Each record has its own unique automatic serial ID since the card numbers themselves will be reused multiple times. There is a field for Card Number, PIN number, Card Issuer (which is related to a table with a list of supervising employees), Card Holder, Card Holder Company (which is related to a table with a list of of contracting companies), Date Issued, Date Returned, and a radio box for if the card was permanently deactivated on return. So each time a card is issued the issuer would fill in all of the information except for the return date. When the card is returned I have a script that allows the issuer to choose the card number from a dropdown on my home page and it searches for any records with that card number which do not have a return date to make it easy to find the card when the card holder returns it so they can enter the return date. What I don't want to happen is for the card issuer to be able to keep forgetting to put in the return date and have the same card shown as issued to multiple people at the same time.
It appears that you have multiple records in FuelCards for the same card. I suggest you consider adding a table to your database where you have one record for each card and then your existing table becomes a record of each time that it is issued, but whether or not you do that is not an absolute requirement here. (But it would enable you to put a portal on a layout where you can see all the records for each time a given card was issued.)
I would guess that your existing drop down is to a global field. The following script is based on that assumption:
Go To layout [FuelCards (FuelCards)] //specify a layout where you can see and edit the date issued.
Enter find mode  //clear the pause check box
Set field [FuelCards::CardNumber ; YourTable::GlobalCardNumberField ]
Set Error Capture [on]
Perform Find 
If [ Get ( FoundCount ) // records were found]
Enter Find Mode 
Set field [FuelCards::ReturnDate ; "=" }
contrain found set 
If [Get (FoundCount) // records for this card exist with a missing return date]
Show Custom dialog ["This card cannot be issued to a new person until the ReturnDate on the record shown is updated to show the date that it was returned"]
Show Custom Dialog ["This card was not found"]