In UiPath Apps, ensure that only one response can be submitted per unique User. If a response has already been submitted for a given User, no additional submissions should be allowed until the current response is processed or cleared.
Use DataService with atleast these 3 columns: Description, User, Status. (Here description might be one column which might hold your form value), just taken as example.
On the page load event check if there are rows with current user and status not equal to processed or cleared.
if it returns rows then display a page for whatever action that you need.
if it returns no rows then display a show message saying you cannot fill the form and throw them to home page or a different page.