Multiple Dropdown / Validation [RESOLVED]

I wondered if anyone had come up with a function for the following.

I have 3 individual dropdown fields. Each dropdown pulling WP post titles.

I would like these dropdowns to be unique as in no duplicates across the 3 dropdowns. Having selected the dropdown it would check the others and validate false if it has already been selected. Preferably to raise a validation error to the user to choose again.

Hopefully this makes sense. Any help would be gratefully received.

How about this approach:

Many thanks Chris.

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