This error indicates that the number entered for a check, deposit or other transaction already exists in the bank account's register. The system does not allow two transactions of the same type to have the same number. Example: Having two checks with the same check number is not allowed.
The message indicates there is a check with that number already in the checkbook register. If the number in question shouldn't have been used yet, do a search for the check number. Once it has been located, the check can be edited or deleted. For instructions on searching for a check number, please see the following link.