csscss by zmoazeni
csscss is a Ruby gem (1.9+) that parses CSS and reports which rulesets share duplicated declarations, on the premise that cutting duplication is one of the best ways to keep stylesheets maintainable and that nobody can hold every ruleset in their head. It runs from the command line on local files or URLs, has a verbose mode and a minimum-match threshold, and also handles Sass/SCSS (with an experimental flag to ignore mixin-generated matches) and LESS. It deliberately reports duplicates rather than removing them automatically, and there are Compass, Grunt and Gulp integrations plus contribution and bug-report instructions.