This will keep happening though, and here's why: New people come to the sub every day. Most of these people don't know what a new queue is, don't know why they should be bothered to search if someone else has the same post, don't give a shit about reposting or if you've been through this before. They have a thought, which they believe to be their own personal revelation, that they want to find support for to validate themselves. Or they have just acquired or made something from the show and want to show it off, regardless of how similar it is to what many other people are doing or buying. How many Lucilles have we seen, really? Myself, maybe 50-60 just since the S6 finale. I've lurked this sub for years before I made an account - how many cakes have I seen? Too many (but every now and then there's a really good or novel one). Other people's posts don't generate karma for them, either.
There are several solutions, other than arguing with them, which will generally go nowhere:
Report these posts to the mods as reposts or shitposts and let them decide;
Downvote them and move on, i.e. don't feed them;
My personal favorite: set your preferences to "don't show me submissions after I've downvoted them", then start at the beginning of the new queue and downvote everything you don't want to see again.
Downvoting in this way is fair because they add nothing of "value" to the sub, which is the criteria for downvoting, and "value" is a subjective judgement.