To duplicate a transaction:
Click the down arrow at the right hand end of the transaction.
Click the duplicate icon (next to the trash icon) in the bottom left of the edit transaction box.
A new transaction will be opened and pre-populated with the date, description, amount, category and GST status of the existing transaction.
You can then either edit and/or save the new transaction.
Note: The newly created transaction will have an account of 'Manual transactions'.