In Mathematica I have a list:

x = {1,2,3,3,4,5,5,6}

How will I make a list with the duplicates? Like:

{3,5}

I have been looking at Lists as Sets, if there is something like Except[] for lists, so I could do:

unique = Union[x]
duplicates = MyExcept[x,unique]

(Of course, if the x would have more than two duplicates - say, {1,2,2,2,3,4,4}, there the output would be {2,2,4}, but additional Union[] would solve this.)

But there wasn't anything like that (if I did understand all the functions there well).

So, how to do that?