This topic is beat to death with no real answers on how to fix it, because at the end of the day there is always a “work around” or it’s “unfair”
Here are 5 Similar Threads
Policy suggestion; Copying Clothing for sale be against the rules
"[How To] Stop Clothing Copying"
https://devforum.roblox.com/t/stealing-clothing/12566
Max 1000 Clothing items Account without manual approval
https://devforum.roblox.com/t/clothing-copies-and-tags/95124
As far as checking exact duplicates, Roblox does this already for verification reasons. If you upload a Decal that has already been uploaded and accepted before, it auto-accepts. (Same for Decline.)
The issue is, there is a lot of the Roblox Clothing Template that is blank space, so you just change a few of those pixels, and boom, upload.