It's not just about fairness to the singers, it's about fairness to the rest of the company. A duo/trio/quartet doing 2/3/4 times the usual quota of songs (together) will get to sound a bit samey, while the aim of most singarounds is to have variety.
I have to say, though, that groups like this are likely to be better rehearsed and presented than some club singers, and this may be the reason that MCs sometimes give them what seems like more time than their due. It also depends on how many other singers are there - on a thinly-attended night, the arrival of a half-decent duo or group will be very welcome.
I think most groups appreciate that arriving mob-handed does upset the balance of the session, and are happy to be given whatever time can be allocated to them without upsetting the rest.

Marje