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A dashboard cannot be that dynamic, so you'll have to look into your idea of tagging duplicates per your comment. If you don't want to go the route of actually making the field unique - you could use a formula query to check the phone number that the user enters and check for existing records and if the count is > 1, meaning that phone # is already in there, then pop up a message saying that this phone number has already been entered and they should consider checking it before saving.