Part of Luhmann's practice in looking up ideas to place in his zettelkasten first was a means of preventing duplication of ideas. If an idea is repeated, that can be noted on the extant card as evidence that others see the idea too or one can compare the potential subtle differences as a means of expanding the space.

Eliminating duplication also assists in the ratchet effect of collecting information and connecting it.

reply to u/Flubber78769 at https://www.reddit.com/r/Zettelkasten/comments/17ajd34/ive_been_struggling_with_duplicate_notes_within/

This is the value of actually indexing your content. You can do a quick search around the index entries which provides a natural check against duplication, but importantly it'll let you think about those ideas again and spend your time more profitably by expanding upon them instead.

Occasionally I'll find duplication from one source to the next which provides some support about an idea's value or spread over time, especially when I'm tracking usage of a thing, so it's not always the case that duplication is automatically a bad thing.