There are 625 questions tagged with fiddler and 17 tagged with fiddler2. Both are references to the exact same software. "fiddler2" is the name for the current version of Fiddler under .NET 2, while "fiddler4" is the name for the current version under NET 4.0, but in both cases the actual program is called "Fiddler".
The tag wiki for fiddler2 already has a proposed edit recommending it not be used.
It should probably be merged with, and made a synonym of, fiddler.
As suggested by j0k I have retagged all the existing fiddler2 questions. I don't have enough tag score to suggest synonyms but I think it would be a good idea for fiddler2 and probably also fiddler4 to become synonyms of fiddler.