If you are using project approval and manager approvals, the recommended practice for managers who are also PMs is to process the manager queue first in order to reduce duplicate approvals.
If the user approves the Manager queue first, then anything in the Project queue will be automatically approved. If the user approves the project queue first, they would need to then additionally process the manager approvals. This is because the manager approval queue may have timesheets with other projects, like PTO or billable projects that do not require project approval. For this reason, if a Manager is also a Project Approver, they should approve items in the Manager queue first.