Duplicate photos of entire photo library

In Photos on my MacBook Pro, I switched the preference for iCloud Photos from "Optimize Mac Storage" to "Download Originals to this Mac". Now, I have two copies of every photo in my entire photo library!

Is this supposed to happen? I thought this was a selection to choose between either original size OR an optimized size. However, it seems that I now have both the original size AND optimized size version of every photo.

How do I get rid of the optimized size versions? It would literally take days to delete them manually. How do I even tell the the two versions apart? The duplicates have the same time stamps.