You were asking for data validation
The value in D10 ( using a formula ) returns FALSE. Entered in the Data Validation wizard it would trigger an error. Thus FALSE is correct
As for D1, you are right I forgot the exact match switch =AND(MATCH($D6,ID,0),(COUNTIF($D$6:$D6,$D6)=1)) which you can use uin the DV now
Your solutions of course work very well so thanks to both of you.
However I think I will go with Pecoflyer's solution as it also works and I understand the formula immediately. I have never been that good at VBA and I am just starting to learn about LET.