Don’t be able to add duplicate values in a REF element type

I have a form where I can add values from another table using a REF element type.

I want the user to not be able to select the same value (hide the value from the list if he/she already selects it) I believe this is possible using a Valid If formula, but I’ve been trying to create the expression without success.

Here’s a screenshot of the problem I have:

I want to know if this is possible. If yes, I appreciate if someone can help me :slight_smile:

Thanks!

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