Today’s invention is a bolt-on feature for YouTube comments (although this still looks like a great option to me).
When you start typing your witty, well informed remark, an automatic search is performed of the comments section for the three comments which best match what you are writing.
If your pearl of wisdom has essentially already been typed by someone else, then at least you are aware of that before persisting in sharing it with the waiting Interwebs. With any luck, this will limit the number of duplicated comments.
In a more draconian mood, I’d be inclined to use this search/comparison tool to simply delete any comments which were found to have been substantially repetition.