It is probably because they have two leases expiring the same time. This can be caused by doing a Renewal incorrectly.
To check, go into the Resident ledger, and look at the Lease History module. If it has two lines with the same Lease-End date...that is what is causing the duplication. They should not have two leases that expire at the same time.
Change the Lease Ending date on one of the leases and they will no longer show up twice.