Is there any benefit to capturing multiples of the same species? For example, say I've captured a Sectoid and researched the related interrogation, and now I've captured another - does anything happen?
A few hours into the game, I can't answer the biggest part of the question ("What is Captive [Alien] good for after associated interrogation is done?") except to say that you can't sell them, so I suspect they must serve some purpose down the road, however menial.
However, I've noticed that in the case of Sectoids, for instance, I often get an undamaged Plasma Pistol along with my captive, instead of generic weapon fragments.
Seems like there are pros and cons to capturing instead of killing aliens after the first of each type.
You only ever need 1 live alien of a particular type, with the exception of outsiders, of whom you need 2, but they produce items that don't require Alien Containment.
To turn your surplus live specimen, of whom you will likely have a huge buildup if you like free plasma weapons, into useful cadavers, simply demolish your Alien Containment facility - all live specimen will be killed and added to your "inventory". The only downside here is that you will need to reinvest the $85 and 7 days before you can capture new aliens.
Note that you can't find out how many live aliens there are in your base.