How to prevent duplicate entry in google sheets per row

by user1227124   Last Updated October 16, 2018 15:03 PM

So using the example data below, how can I prevent a duplicate entry with an informative message. The duplicate entry should be based on the row and not the whole sheet.

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So, if I was to add a second column named Colour2, and I added 'Blue' for Bob. I want it to prevent the entry with a suitable message. But I dont want it to prevent the entry of 'Blue' for Alex as he hasnt chosen Blue yet.

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