Below approach may help you for your requirement .
1. Load the previous case data in data page
2. On submission of new case, we can validate data of existing case from current data page with old data page via data transform and if there is duplicate case value then you can configure error message as per your requirement.
You can have a look at Duplicate Case functionality in Pega. Ideal way would be to use a Case match rule to check the duplicate on submit. As doing it on change of every field may not be an ideal solution for long term.