Approval Workflow - Limit User Approval

I have setup my approval process setup based on two routing (two approvals per voucher required). First level routing/approval is by department and second level/approval is by the monetary approval. I'm having issue setting up my workflow to skip a user who has already approved the voucher at the department level. For example, Dolly can approve IT department invoices and invoices over $500. A $550 IT department invoice is entered in PS. First level routing (department) routes the voucher to Dolly. Second level routing (monetary amount) routes the voucher to Dolly, Sue and Amy. How can I setup the