Merge CSS files

Not too Plone related, but maybe someone can give some advice anyway:

When 'Plone or theme CSS' defines some CSS-class properties and the same is defined in CSS-files from an add-on:

Is it possible to merge the CSS into one CSS file avoiding duplicates?
Even if it has to be done manually, it would be helpful

For example: for one site I discovered that

.fa-arrow-up:before {  content: "\f062" }

Is defined in 'bundle-leaflet.css', in my theme.css, and in font-awesome (add-on) css (of course, it might be that more css is defined in the other css-files, like color or font-size).