No duplicate values in two or more forms within the same application

No duplicate values in two or more forms within the same application

Hi...

I need a little help.
I have an application with more forms inside.
I would like that a field become unique in every form (i don't want that a student duplicate a value).
For exanple:

A student must book for an activity for the days 6 and 7 May but he can't book himsle for the days
10 and 12 May because he already booked for the days 6 and 7.

Ho can i coul do it ?
Thank you in advance for every suggestion.