Thanks, let me rephrase my problem:

I need to enter data in columns H where this formula is needed and column G where criteria has to be met. Column G for example has rows for data validation from 2 to 19:

First criteria - H should not accept the data (which is numerical) if the value in G2 until G19 is duplicated. The value in G column must only entered once

Second criteria - H should not accept the data if value of column G is blank.

I hope this is clearer now. Thanks.

=COUNTIF($G$2:$G$19,G8)=1

but, since G2:G19 intersects G8, for G8 to be repeated, shouldn't the check be:
=COUNTIF($G$2:$G$19,G8)>1