I have been grinding Mendi King for the new greatsword, and have only gotten a single fragment because it instead gives me fragments for the armor that i have already fully built.
This has always been the case, and missing out on loot because it instead gives you something you already have, isn't interesting, it only serves as a frustration in the grinding process.
The ideal would be to implement a system that recognizes the fragments you already have, and doesn't give you duplicates. But if these redundant fragments must stay (To make the loot feel diverse or create incentives or whatever) at least implement something akin to a mercy system where you can either sell them, or turn enough of them in for something you actually want/need