Fix duplicate styles in CSS output

[pioug]

I think this PR will fix #49 #55.

I was going through the issues and realized that my build in my project has repeated styles too.

https://accounts.bandlab.com/web-accounts/scripts/app-033141d7.css 👇

The number of duplicate copies of the css seems related to the number of dynamic import() statements used within the specific bundle. - #55

This was a good guess. I found a fix and added a test. I probably introduced this bug 🙈

Without the fix we would get:

.rollup {
  color: green;
  user-select: none;
}
.rollup {
  color: green;
  user-select: none;
}

[thgh]

Published as 4.5 🎉

[pioug]

@thgh Can you double check the v4.5 release when you have time? When I look in the /dist on https://www.npmjs.com/package/rollup-plugin-css-only?activeTab=code, I don't see the updated version with the usage of Set(). So the bug still persists for me!