If you see the following message, there may be a few causes:

"This recording is already open in another instance of FlashBack.  It cannot be opened for editing twice."

The obvious cause is if the recording genuinely is already open in another instance of the Player. 

However, you may also see this message even if FlashBack is not already open. In cases like this something else is accessing the recording file. The most common example would be if the recording is in a location that is synced to the cloud (using Google Drive, etc) and it is actively being synced, which gives Windows the impression that it is already open.Either wait for the synchronisation to complete and then try to open the file or make a copy of the file outside of the synced folder and work on that copy instead.