Oracle - Row duplicates in a table with a PK loaded by sql*loader

I had an interesting problem today.  One of the developers told me a nightly job had stopped with an error.  I check the usual things like has anything changed and was told no.  The error was sent to me and was the following:
SQL*Loader-926: OCI error while executing delete/truncate (due to REPLACE/TRUNCATE keyword) for table REPORT_TEMP
ORA-01502: index 'PK_REPORT_TEMP' or partition of such index is in unusable state