UPDATE 2 pm:
Google reports that the problem with Google Drive has been resolved.
UPDATE 1:30 pm:
Mitigation work is currently underway by Google’s engineering team.
Google does not have an ETA for mitigation at this point.
Drive Notifications has been resolved for some of the affected users.
Since yesterday users are reporting and Google has confirmed that when sharing with specific recipients in Drive (and related editors) recipients may receive more than one email notification. Google engineers are working to fix the issue and expect resolution shortly.
Status updates on this issue can be found on the Google Apps Status Dashboard.