Can't find the button for duplicates in Photos!

I'm sorting photos and have emptied mobiles, cameras, etc. and dragged them into the Pictures program on the Mac computer. I know there are a lot of duplicates and instead of going through all the thousands of photos I want to search for the duplicates and merge them. There should apparently be a "duplicates" button in the side menu, but this is not visible to me and when I search får "duplicates" in the program (Pictures) I get 0 results. What could be the error?