Pega uses pr_extract_time table for tracking the last time when the extract is executed by referring to pxLastUpdateDateTime column. This column is updated only when the entire Batch Process is executed successfully without any interruption. Even though if some of the InsKeys failed to extract due to any data issues also as the batc is completed it will be updated and the failed ins keys would be written to Pega-BIX log file.
In your scenario as the server outage happened the process is incomplete which means pxLastUpdateDateTime will not be updated in table. So there is no chance to extract the data from where it is stopped. Extract should be re-run again and once the data is retrieved successfully duplicates should be deleted using duplicate delete Query that Pega provides.