Preventing duplicate selections

Let assume you have Company List sheet as

and in each employee sheet (in the sample from A to C) data starts exactly from A5

With that we may generate spill for validation which has values from Company List not repeating in employee sheets

=UNIQUE( VSTACK( TOCOL( $B$5:$B$70, 3), TOCOL( VSTACK('A:C'!$A$5:$A$70),3 ) ),,1)

and use this spill in data validation as

Please check in attached file