validation for dropdown

[gapper]

Hi,
I have a dropdown into a repeteable group.
How can I avoid that the same item value is added twice in the same form from the same user?

E.g.: The voices of a dropdown are: item 1, item 2, item 3. The dropdown is into a repeteable group. How can I avoid that the same user adds twice (or more obviously) item 1 for example.

 

[rob]

The only way would be to write a php validation to check for unique values on submit

 

[gapper]

Please could I know the php validation if it were a snippet code?
If it is not or for any else reason it is not possible to know it, please don't worry. it is ok.