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SUBMIT YOUR IDEAI didn't realize you could group by other columns in the Multi-Row Formula tool. That's a great tip. Here's what my solution looked like before I checked out all the comments and everybody else's solutions
Mine is pretty similar to the given solution (notes in spoiler).
My solution:
Here's my solution:
One question I had was how would you handle this if you weren't just looking 1 row back, but at the last 'non-duplicate' click, since the number of rows back could be variable. Any suggestions?