When a report is generated it is important to highlight that dependent on how you've created the report it may pull through duplicate vacancies.
This can happen if:
1) A vacancy has been posted then un-posted then re-posted.
To avoid the duplication simply apply a filter called posting status and then choose the option is not equal to > you can then choose "unposted". This then means that only posted or expired vacancies generate to your report.
This filter option is available in both live listing and ad-hoc report functionality.
2) You are posting the vacancy to more than one source; such as the internal and external site.
To avoid this duplication then simply apply a filter called sourcing type and the choose external site if you only want to show vacancies posted to the external site, internal site for internal or redeployment source.