Good idea to display that filename.
The same thing happens on my machine. I'm thinking that on reset, the intent is to set the filename to be the home folder (or null), but for some reason, it's being concatenated with itself. In lieu of it being "reset" to whatever is specified in SF-FILENAME, if it were set to the home folder (or null), everything should be OK, wouldn't you think?
When I set the filename "" to an empty string (null) in Scheme, it displays the home folder. Perhaps on reset, somewhere in GIMP the filename is set to null (which makes it the home folder), then it appends the home folder again, thinking it's being appended to an empty string. That's how it gets doubled. I dunno. It's just a theory, based on what I've seen so far.
Do you think it's worth changing
gtk_file_chooser_set_filename() to
gtk_file_chooser_set_current_name() to test what happens?
I was under the impression that hitting the reset was the same as "refreshing scripts", except it was just being applied to the current script. Obviously, that's not the case. I guess the code that's executed when a reset occurs needs to be traced to find out what's going on.
Btw - I installed Fedora 20 Gnome3 in a VM and got the same errors there too, so it's not a GTK3 DE issue.