Under the File menu,   "Save as copy" - with simple settings of appending something such as _copy to the filename,  or asking the user to rename.  Perhaps a timestamp option as well?  With a hotkey that applies the default setting. 

 This is a feature I find very handy in the GIMP,   sometimes I don't know where I'm going with a particular idea and like to "snapshot" various incarnations along the way in a manner that makes them easy to sort thru (Undo/redo trees should have branches).   No doubt it'd be handy in other ways as well.

Thanks for Bluefish and all the wonderful work you guys do.   I'll be donating soon

 Andy G
Spearfish, SD, USA