How can i add custom validation while submitting a request in catalog in OIM 12c

Product name - Oracle Identity Manager

Version - 12.2.1.4.221013

Sample scenario: 

User1 has a request submitted for ABC user for XYZ account to be provisioned. This request is in request awaiting approval.

User2 submits a same request for ABC user for XYZ account. 

This creates duplicate requests in the system. I want to stop the second request by adding custom validation on Submit button in catalog.

I have heard that we can achieve it by writing custom bean class. Can somebody please let me know how to write a custom bean class and link the same to Submit button.

Thanks in Advance.