finding all non-unique lines in a file

I'm trying to use uniq to find all non-unique lines in a file. By non-unique, I mean any line that I have already seen on the previous line. I thought that the "-D" option would do this: -D print all duplicate lines But instead of just printing the duplicate lines, it prints *all* the lines when there is more than one. I want to only print the second and subsequent copies of a line. How can I do this?