Data validation rules are triggered when a user adds or changes a cell value.
The AND function takes multiple arguments (logical expressions) and returns TRUE only when all arguments return TRUE. In this case, we need two conditions:
Logical 1 tests if the input is a number using the ISNUMBER function:
The ISNUMBER function returns TRUE when a value is numeric and FALSE if not.
Logical 2 tests checks that the input doesn't already exist in the named range "ids":
COUNTIF returns a count of the value in B5 inside the named range ids (B5:B9). If the count is less than 2, the logical expression returns TRUE.
If both logical expressions return TRUE, the AND function returns TRUE and validation succeeds:
=AND(TRUE,TRUE) // validation successful
If either logical returns FALSE, data validation fails.
Be aware that numeric input includes dates and times, whole numbers, and decimal values.
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.