Hi ede,
I do not know either. I suspect that living in /snap/bin/duplicity as a link to /usr/bin/snap may be the only indicator.
It's possible that there are environment vars active when under snap. It's hard to tell.
As for the snap itself,
- the python3 interpreter is included internally,
- is always Python 3,
- the shebang line is correct,
- script is marked executable.
If you absolutely must parse the command line, duplicity should reside at
/snap/duplicity/current/bin/ along with python and python3 for the snap. Snap runs first to set up a virtual env around that python3. Messing with that could cause some real twitchy bugs that I would hate to chase.
BTW, this is how duplicity snaps run under Ubuntu 16.04. If it weren't for Python 3 under snaps we'd be out of luck.
...Ken