An Evil Twin in ClearCase is a term used to describe two different elements (files or directories) that the developers conceptually recognizes as different versions of the same elements, but whereas ClearCase recognizes the same two elements as two completely unrelated elements that just happens to share the same name.
The short version is, that:
Every time a user expected to see a 'check-in' menu item but instead finds a 'add to source control' menu item and ignores the fact that the check-in is missing and goes for the second-best and adds the element ...again!
In most situations Evil Twins are unwanted! And typically the ClearCase administrators will have installed triggers that prevents them from being created. The Add to source control operation is simply cancelled if the folder that you are about to add elements to has a previous version (anywhere in the version tree) that has already used the name you're trying to (re)introduce.