If you use Family Tree Maker, there is a Sources page that will list everyone attached to a single source. I had done what you did - created a duplicate - and was able to find and fix them by identifying them in my downloaded tree in Family Tree Maker, then make the changes online and re-sync my tree. I use FTM as a backup and make all my changes to my online tree.
I recently changed the names of all the sources that *I* have created and put an underscore "_" in front of the name, so they all sort to the top of the source drop-down list now. Much easier than trying to remember what I had called them and having them buried in with all of Ancestry's sources. I identified all of them via Family Tree Maker's Source page.