It often happens on discussion boards. Sometimes people get caught when they forget who they are pretending to be and post under the wrong account.
It's sometimes easy to spot - eg when a group of 'newbies' all start saying the same thing (often picking on one poster or a group of them), and have an obvious knowledge of the board and its members. They often start posts claiming to be a 'long-time lurker', and there are other giveaways. There are certain linguistic 'tic's that can give people away, too.
It can be banned members, but some people just enjoy it, or post in defence of a friend who they think is having a hard time. It gets worse in the school holidays, too.