Removing Duplicate photos in bulk

I had an apple senior advisor help me remove 900 duplicate photos at one time. When I try this on my own, the duplicate function in the photos app forces me to approve the merger of each photo, one at a time. I know he used a different method than the duplicate function in the side bar within the photo app. I should have written down what he did. What key strokes should I use to replicate what he did? I am running Sequoia 15.3.1 on an iMac. Thanks, I know it can be done:)