It sounds like you are already in the situation, so I would suggest:
- use the studio "Analyze solution for code clones" feature
Use the new feature in Visual Studio 2012 called Code Clone Detection.
- enter all the identicals and the top 25% of similar into the backlog
Enter all the identical and the top 25% of similar into the backlog.
- make the folks creating the duplicates clean it up
Make the folks creating the duplicates clean it up.
Do this a few times and people will start to realize they need to be more careful.
If you don't have Ultimate or Premium edition then look at CodeRush or Resharper they have similar features.