Only add imported user once to groups #257
Description
I just ran the importer several times, once for each large space.
As I enabled importing the users/groups, these were properly imported.
However users were added to each of their groups multiple times (every time they appeared in an import, it seems).
This makes for more "click-work" if one wants to remove a user from a group later on.
Suggested improvement:
Only add users to a group if not already present.