So most of us have at some point had issues with duplicates in Salesforce.
Its ok, you are with friends now, we can admit it.
I have answered a lot of pleas on the Salesforce Blogs from people looking for possible solutions, and here is a summary of what I tell them. This is not a definitive list, and I am not necessarily promoting any Apps or Tools above others. But hopefully this can be a useful staring point for you.
Here is a free one that just identifies ‘how bad things are’– Dupe Dive
This is free, a bit of an effort to set up though – Dupe Catcher
Dupe Blocker is a decent paid App
Once it is clean, Unique Entry is great, well worth a look!
Cloudingo is a well rated App, it’s on my Saved list, but I have not tried it yet
Another option is to get a database wiz to look through it and identify dupes that you can then merge or delete as required!