Duplicate Validation in Form Input

Is there a way to check if a form input (eg. Phone No) already exists and shows an error message to the user and not let them submit the form?

They’re isn’t any validation other than marking a field as required.

I suppose it would be nice to have a unique value validation check, or maybe a new data column of a “Key” type, where you could specify which columns combined are part of a unique key for a row, and then perform a validation check if the user ever attempts to add or edit a records where the values match an existing record in the sheet