Orion will allow you to pay two Vouchers for the same Vendor with the same Invoice Number; however, it will warn you upon entry of a Voucher using an Invoice Number already entered on that Vendor.  If you enter a Voucher for a Vendor and enter an Invoice Number that you have already entered under this Vendor, the system will display the following:

Note: Orion tells you the Voucher Number already entered using this Invoice Number.  You may continue entry of the new Voucher using the same Invoice Number without any consequence so if this warning appears, check to make sure the Invoice you are entering is not a duplicate.  This window is the only place you are warned.