MDM Workflow - Exclude a user in a step

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We are in SAP MDM 7.1 SP04.

1. Start - Process - Approve -- Stop
2. Start - Check out record
3. Stop - Check In Record.
4. Process Step owner - Launcher of the Workflow.
5. Approve Step assigned to a Role.
6. 2 roles - Requestor and Approver.
7 User A has both Roles Requestor and Approver.

Q: Requestor(User A) launches the workflow - while sending from Process to Approve Step - we have to ensure that the owner of process tep in the workflow should not be able to approve the record -- Is it possible to check and exclude or show an error message that the previous owner is not allowed to be selected in the Approve step.
so my another question to you is

what is the assignee type and authorizations are provided to this User or Roles ?

are you assigning the same roles to both this users ? What is the Execution type for this Workflow? try to make it as All Sequential Orderd and then see if it works if it does not then please let me know we will check out some other way for this/

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