Dom N. once this starts, you have to clean your project completely of the duplicate classes.
After you are sure that your project is sanitized of duplicate classes - double check that class sync is on.
If you paste and it still has issues, turn it off, save (cmd + s), refresh, turn it back on, save, refresh, and try pasting again.
If that doesn't work, and you are pasting and its still duplicating classes, share your read-only link and where you are pasting components from (and what components) and we'll investigate more.
Apologies for the frustrations here!