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To create a custom function use Google Apps Script. This is very similar to writing a JavaScript function. /** * If the arguments are numbers, the sum of will be returned * If one of arguments are strings, they will be concatenated * * @customfunction */ function myCustomFunction(a,b){ return a + b; } Start by reading https://developers.google....


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Try to input this formula instead of yours. Maybe is just your locale settings that prevents you from using the formula. Try semicolon instead of comma: =COUNTIF($A:$A; A1)<2


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