When you look at the description of the Requests dialog, you’ll notice the parameter exclude_ids
, by which you can specify user ids that will be excluded from the multi-friend selector.
So if you look up in your database which of their friends your user has already send a request to, you can put those ids in there.
Only cave-at: This parameter is not supported on mobile devices; so if you are targeting those, you might have to look for another solution (which would be most likely to create your own multi-friend selector) – but I’m not sure right now whether this restriction only applies to native mobile apps; maybe when using the JS SDK in a web-app it’ll work on those as well (the wording in the docs is not totally clear on that).