New Stack Exchange sites are created through Area 51, if you are not familiar with the process, check out the Area 51 FAQ. Proposals that are duplicates of existing sites show up all the time, and they are (almost always) quickly closed. If all questions that could be asked on a proposed site are on topic on one or more existing sites, then the proposal is pointless.
MSSQL2012 Stack Exchange, your example, is not a proposal that would progress to a full site, anything you could ask about MSSQL2012 you can in either Stack Overflow or Database Administrators. But why does Database Administrators exist in the first place? Well, it does overlap with Stack Overflow, but not completely. If you check the site's FAQ, they welcome some topics that don't really seem to fit Stack Overflow (you'll need to talk with a DBA.SE regular for more details).
But overlap exists, and it's not necessarily a bad thing. There are two ways to deal with overlap, the first is to declare a topic off topic for the one site and direct questions to the other site. This is something both communities should agree on. The second way is simply for the topic to remain on topic on both sites. It's a good enough solution and when a question that could fit both sites appears in one, we simply let it stay where it was originally asked. Unless it goes for a while without a satisfactory answer or the asker specifically asks for the question to be migrated.
Older questions are a bit more troublesome, as more often than not they have completed their cycle in the site they were asked. Even if they become off topic at some point, there isn't really much value in moving them to a new site, especially a younger one. Younger sites, especially betas, are in need of every question they can get, and migration of older questions might seem a good idea on the surface, but it really isn't, for a younger site the key to success is keeping its audience engaged and, well, entertained, and questions that are already sufficiently answered don't really help.