It is possible that during the transition from version 7 to version 8 of APDL some duplicate tail numbers were saved in your aircraft database. This should not continue once cleaned up as described below due to the new storage method used as of version 8.

What you should do to fix this issue is look through the aircraft tail number list and find the duplicates. Choose one of the duplicates and change the tail number slightly to something you can easily notice. Such as change N123AA to N123AA-2, or delete the N prefix. After you make the change, save the new tail number. Now open that tail number again and see if there are still flights associated with that new edited tail number. If yes, tap the flights row and tap each flight that is associated with that tail number. Then change the tail number associated with that flight to the correct (unedited) tail number. Gradually this will remove all the associated flights from the duplicate (undesired) tail number and you can then delete it when there are no flights left. Sometimes you will find that there are more flights associated with a particular tail number than the other. For example, if after you change the tail number you find that there are 8 flights still associated with the newly edited one and only 2 flights on the unedited one, it would obviously be easier and quicker to remove the one with just 2 flights than the one with 8 flights. in that case just make sure to change the tail number back to what you want it to be on the one that remains after deleting the duplicate one.