ORA-00001: unique constraint (APPS.AP_HISTORY_INVOICES_U2) violated

While Purging the Parables related data in the 12.1.3 version, we are facing this issue.

Why it is failing for APPS.AP_HISTORY_INVOICES_U2, if Duplicates are there it needs to be failed for AP.AP_HISTORY_INVOICES_U2, But we do not see duplicates on this table.

INSERT INTO AP.ap_history_invoices_all

      (invoice_id,

       vendor_id,

       vendor_site_code,

       invoice_num,

       invoice_date,

       invoice_amount,

       batch_name,

       purge_name,

       doc_sequence_id,

       doc_sequence_value,

       org_id)

       SELECT  api.invoice_id,

               api.vendor_id,

               v.vendor_site_code,

               api.invoice_num,

               api.invoice_date,

               api.invoice_amount,