Diffusion - maybe fix import spam bug

Summary: Ref T6350. I just set up a repository to import secure.phabricator.com/P and saw an error in my error logs about this, creating a fatal around when we publish feed stories. this is late enough in the editor code path I could see it firing again and again and again...

Test Plan: the first phabricator commit ever only had one duplicated transaction before i applied this patch. its yet to do it again. otherwise, ask some users with the issue to deploy it and see if it fixes things for them too.

Reviewers: epriestley, chad

Reviewed By: chad

Subscribers: Korvin, epriestley

Maniphest Tasks: T6350

Differential Revision: https://secure.phabricator.com/D10729