How Can I Write a validation Rule That Prevent Repeating Value

[accesser]

I am working in a Continuous form.
One control in this continuous form is Combo Box.
As The select new value in this combo box so new record is added to this form, I want to use a validation rule on this combo box so the user will not be able to select a new value that was already selected before in this continuous form.
So please help me writing the validation rule that will prevent repeating the same value in this continuous form.

Regards,