2039910 - Model User Request for an existing User ID give an error

In Model user request, when a new account request is raised for an existing user ID, system allows you to submit the request. On contrary, when the same thing is done through normal access request, system gives an error message, stating 'User XXXX already exist in system : XXXXX'

GRC 10.0 
GRC 10.1

Maintain Provisioning settings, Provisioning settings, Error, 'User XXXX already exist in system : XXXXX' , Model User Request creation error,
Same existing User ID , GRC 10.0, Request for same user ID , KBA , features and functionality , GRC-SAC-ARQ , Access Request , Problem