Undo the Last Step for importing the attendees
1. Go to attendees page and click "History" on the right upper corner.
2. Here's the Import History Page for checking the import record. It shows the record of importing details (Import time and number of attendees) on the list below.
Click on "Undo" on the right hand side of each record.
Here's the following page for processing the undo importing records.
3. Press "Click here to refresh" to reload the page.
Here's the page when finished processing undo import history.