If you have a bill that's a duplicate of another, you have the following options:
If the bill is in "Draft" status: click on the bill and select "Delete" to delete it from the dashboard. It won't show up anymore.
If the bill is in "Awaiting Approval" or "Awaiting Payment" status: click on the bill and select "Reject" (or "Void" for imported bills). For bills that weren't imported, your vendor will receive a notification and the reason you provide when rejecting the bill.
If the bill is in "Paid" status, contact your vendor to ask for a refund.