I have been running a function in Mathematica 10 with the command,
DeleteDuplicatesBy. The function was working great until one day I started to get an error with the
DeleteDuplicatesBy command. When I tried to run it, the command would turn red and say, "A symbol occurs in more than one context: One of the definitions is shadowed."
I have Mathematica 9 and 10 installed on my computer. I am not sure if that could be causing the error. Here is an image of the error in action.