Each post in Liftoff are marked with "via {instance-domain.tld}" so you would know which account will be associated with when you upvote or comment on it.

There's also the ability to switch instance at any moment, when viewing the post or community, so that sometimes when you found post from All feed but want to comment a post or subscribe community using other account.

If you have overlapping communities subscription in your multiple accounts, you could be seeing duplicates of the same post, but I think that was addressed in the last update as I don't seeing it anymore, at least before my instance updated to v0.19 where it made Liftoff no longer working for me. The duplicates post may appear more in All of all accounts. There's also Local of all accounts, but I rarely use it.

Yeah it seems like the multi-community feature, but this is an auto-populated multi-community feature instead, like the Subscribed community.