Set up avoid duplicate problem

Dear All

Encl pls find the excel file for your reference.

I have problem to avoid duplicate in Column F.
The result of Column F is combination of Column B, D & E

I have no problem to set up the data validation on column F .
If I type manually on Column F, they have no problem to show error message & refuse the data;

However, it does not work when I change the value on either B, D & E even the result are the same on column F.
Can everybody can assist ? Tks in advance

Rgds / AL

Tks Brenie - you are the best !