The transaction is attempting to create a record that already exists. 
This is most often a result of an issue during a previous attempt to process the transaction which resulted in partial completion of the processing, but was then aborted without being saved [due to a workstation crash or power failure, for example].

If you know the details of the transaction, select "Yes" to delete the transaction when prompted by the error message, and manually enter the transaction in The Raiser's Edge if needed.

If you need assistance determining the details of the transaction:
Click Chat with Support and reference this article