We have seen this as well, it also happens if you sync a user into WS1 and then move the OU they are in and sync again. We find that that will actually create a duplicate record.
We have spoken to support about this and the reason for not removing (as told to us anyway) was that it was considered 'dangerous' to automatically remove accounts as you could accidently remove your admin account and therefore no longer be able to access the console. To me that is a little bit like disabling something that could happen 1/1000 but also prevents something that could happen daily in large enough organisations.
I have on my list writing a script that will go and remove those users. It will confirm with AD that they have ben disabled/removed and then remove them from the console. Its pretty low on the list though so dont know when ill get time to get to it.
