So just to get it right: 1. it means that there is only one instance that processes commands and 2. this instance checks all command ids if they appeared before (so keeps track of all ids in an efficient way)?
Not necessarily. Depends on how you shard the aggregate/streams. Since aggregates _by definition_ maintain their invariants, you can have literally a shard for every aggregate. Overkill, but it illustrates the point. Since there's no state shared between aggregates, you can process each aggregate/stream on its own instance.
> this instance checks all command ids if they appeared before (so keeps track of all ids in an efficient way)
Yep. If you trust the client and don't mind losing old commands, this isn't too hard - use a monotonically increasing number (e.g. ms since the epoch) as part of the commandId. Any command with an id less than the commandId in the aggregate will be rejected as "out of date". Note that this serves as an optimistic concurrency check.
If you don't trust the client (or you don't want to lose old commands issued on a possibly out of date view) and just use GUIDs as commandIds, then yes you'll need to keep track of some ids - but not necessarily all. Do you really need commandIds past 1000? (Depends on your domain.) You'll need to "roll up" the events into some summary view anyway to check/maintain business invariants, so keeping, say, the last 100 commandIds as part of that summary view is simple.
Now I only wish there would be a good alternatives for when you want to have multiple instances per aggregate (even if you may have to give up certain guarantees).
If you're trying to do multimaster db storage, like a phone app syncing with some cloud server, where an aggregate resides in two separate locations, this is where event sourcing shines. It's just like git, right, and so you can do all the stuff that git can do - merge event streams, rebase events... it's actually the reason why my pet project switched from an RDBMS to event sourcing. Related video: https://skillsmatter.com/skillscasts/1980-cqrs-not-just-for-...