Data validation rules are triggered when a user adds or changes a cell value. In this example, we are using a formula that checks that the input doesn't already exist in the named range "emails":
COUNTIF returns a count of the value in C5 inside the named range emails (C5:C9). If the count is less than 2, the expression returns TRUE and validation succeeds. If not, the expression returns FALSE and validation fails.
Note: Cell references in data validation formulas are relative to the upper left cell in the range selected when the validation rule is defined, in this case B5.