De-duplicate member-added messages with securejoin

[hpk42]

with securejoin it's irritating and noisy for users to see system messages about "X added Y" multiple times if "X" has a multi-device setup and posted an invite link somewhere. We can not easily co-ordinate on the "adding side" but we should be able to de-duplicate/skip the "member added" system message maybe just on the condition that the user is already a member.

[iequidoo]

The "Message-ID" solution from #5356 should also solve the problem and works even if Y is removed before another vg-member-added is received. Otoh the @hpk42's solution works for members joined between vg-member-added messages which can easily happen if multiple members are added at the same time, so maybe implementing it first makes more sense, and it should be easier.