Nis (applications?) has a line limit in the "group" entry, so you can't
have very long lines (=many users)
The way to solve this is to put aliases in the group map, like this:
MY_GROUP::123:user1,user2
MY_GROUP::123:user3,user4

This is supported by the OS, grp.getgrall(), etc, but
nis.cat("group")
only puts the last entry into the dict it returns.

I guess the best thing if you want to return a dict is to merge the
members if both the name and gid matches.

Can you dig up some specification for that behaviour? The more official,
the better.

Otherwise, I'd rather add an option to return a list instead of a dict;
people wanting that semantics could then do their own processing.

Here's another description of the issue:
http://www.tldp.org/HOWTO/NIS-HOWTO/maps.html#AEN548
There's a suggestion to have different group-names but same gids, but
automatic splitters of nis-groups doesn't allways do this.

Right, grp.getgrall() returns a list, that's why it works.

Is this really a bug (discrepancy between nis.cat doc and behavior) or a feature request?

I think this should be an enhancement request.

nis is deprecated as per PEP-594, so there won't be further enhancements to it.