I have been trying and trying and CANNOT find where this is. The rightmost column for me on mac only has two icons at the top, an eraser, and a little box with an arrow pointing down. Neither come up with a menu that allows for merging duplicates. I've searched on the web, too. If I could leave a screenshot in here, I would. I'm using the most up to date version of Z.
See https://www.zotero.org/support/duplicate_detection