How can you identify duplicate CSS rules?

Here is one possible solution:

1. Copy both files into a CSS code formatting tool e.g. http://www.codebeautifier.com/
2. Format it so each CSS ruleset is on one line, e.g. p { font-size: 13px }
3. Put the result into a sorting program, e.g. the sort command on Linux/Mac terminal. There are online tools that can do this too.

Now all the duplicated selectors should be next to each other. You should be able to combine them by hand pretty easily.

Also, don't forget that different ordering of CSS rules can have different results.