Install Node JS
https://nodejs.org/en/download/
if you have already node js installed or after installing open node command prompt window by typing node( on windows machine) in start.
Type following command to install css purge tool
npm install css-purge -g
After the tool is installed,
Open the folder where node command prompt window is open and paste messed up css file there and then type following command in node cmd prompt window
css-purge -i style.css -o style_purged.css
where style.css is the name of messed up css file, the above command will create a new css file with name style_purged.css which does not contain duplicate css rules.
But be careful, it will also remove the comments you have.
https://www.npmjs.com/package/css-purge