voucher posting abends on Step IU_PROCESSOR.U400.U400-010

Hi

When voucher posting is run , it abends at Step IU_PROCESSOR.U400.U400-010 with unique constraint error. We noticed that the issue is occuring again and again for the same voucher.  Could you please tell me how the same voucher be picked up by voucher posting multiple times ( This is part of batch schedule and every day this voucher is picked up) . Also noticed that this voucher is not appearing when i select post voucher option in voucher posting run control page. But some how some sql in AP_PSTVCHR  is picking up this voucher and trying to process. Any inputs