We very often see the same username created across many instances

Guilty as charged. I'll say though, there are several legitimate reasons why one might want to do this. I personally use it as a substitute for Reddit's multireddit feature, by grouping community subscriptions across different instances by theme. As long as users use the same username across instances I don't think this practice should be automatically regarded as an attempt to sockpuppet. It that was the goal, the accounts would definitely not be using the same username across all the instances.

I didn’t mean federate in the Lemmy/Fediverse sense, but rather in the collation/multireddit sense. In other words, gathering posts from many different sources, in this case instances, and displaying them as a single list. Given the context, calling this “federation” was probably very confusing, and definitely a poor choice of words. I apologize for that.

In Liftoff, this takes the form of an All/Everything view (where “All” means all currently logged in instances) which collates and displays posts from many different Lemmy instances at once. This is the feature I was looking for in other clients.

In this context it probably is, since the federation that is done on the fediverse is something else entirely. “Collate” might be a better word, perhaps.

Workaround