Merge is supposed to also merge duplicate sources. However, it has one fatal flaw: It will only merge sources if there are exactly two of them. Three or more copies: no merge. Once you have accumulated three copies of a source, by three successive Appends, perhaps, there is no turning back. Their numbers will grow like topsy. 

So, how to rid yourself of them? 

1. Most of them are probably unreferenced, so it pays to try to rid yourself of them directly. 
1. Click Edit/Edit Master Sources. 
2. For each source, click Delete. If there are references, you will be told. Go on to the next source using the navigation arrows at the top of the dialog screen.
2. If there are no duplicates among the remaining bona fide sources, you are done. 
3. If there are any triples or more of duplicates remaining, change their titles so that there are only pairs and singletons. You can add suffixes of "a", "b", "c" &c to accomplish this. 
4. Click OK, People/Merge Duplicate Individuals &c. This will merge the paired sources. If there were lots of duplicates, you will have to repeat the above steps to create and merge more pairs. 

After there are no more duplicates, go back and remove any suffixes you may have added.