Checking for missing properties in a form that updates a case

I have a form which updates a case.

One of the properties in the case is the GPS location of the case (VBA_gps).

In the form I want to skip the question asking for the GPS location if the
property already exists (ie is not blank or a missing value).

How do I do this?

I’ve tried to use a hidden value to hold the value of VBA_gps when the form
is loaded (VBA_loc) and I’ve tried using this hidden value as a Display
Condition to determine whether to skip the VBA_gps question (or not) but I
get a ‘circular reference’ error when deploying.

Grateful for any advice.