I had the same thing happen to me a year or so ago, duplicate brushes and what I did was hold down the "ctrl+alt+shift key when you start up elements. That can be tricky because you have to hold those keys down just a the right time. Took me about four tries. You have to hold them down just as it starts up, you will know you did this at the right time because you will be asked if you want to reset preferences or something like that. I think this is solve your problem. I wish the elements guru's would jump in here.
another thought..
if you know which ones you want to get rid of?
Change your view to list and highlight the one you want to delete, right click and pick delete.
Just make sure you are getting rid of the correct ones.
(click the little circle with the triangle on the top right corner of your drop down brush menu to change to list.)