Command-a (⌘-A) selects all.
Open the Duplicate view in Utilities, do ⌘-A to select all, and click on Merge. Like picture deletions, the removed pictures will go into Recently Deleted for 30 days in case you want to examine them and change your mind.
Are you sure you want to do all of them without looking at each one? I wouldn't. Some pictures may not be exact duplicates, but similar enough that Photos brings them to your attention. For instance, here is a duplicate pair of mine:

I like both of them, and one would be lost if I just selected all and blindly hit Merge. If I were you, I would look at each pair closely before deleting them.
Be careful.