Verifying that a specific question always is a unique response


I am developing an app that will be an online version of a paper form.
This form has a unique identifier and I would like to add it to the app.
I would also like the app to notify the recorder if the number has been
previously been entered, Is there a way to do this?



I do not think it is possible to make a sequencing numbering, as the
(offline) device would not know if a form has been submitted by
another/device and user.
In some cases you could make a hidden value based on several parameters
like device ID, user ID, date , time. based on this you could index. If you
do not need indexing but just unique numbers, commcare also makes an ID on
the background.