Aggregator (sometimes) adds feed items multiple times if custom process hook is active.

Hey there,

I noticed that if you activate a custom implementation of hook_aggregator_process
and then proceed to trigger multiple updates of a feed, you get some feed items multiple times.

Steps to reproduce:

1. Activate additional hook_aggregator_process implementation at admin/config/services/aggregator/settings
2. Make sure the feed has changed since the last update or remove all items
3. Click the "update items" link at admin/config/services/aggregator like a maniac
4. Wonder where all those extra feed items came from

It might be noteworthy that this is happening so fast that even though I check if a certain guid already exists
in my table and the check yields a negative result, that often already has changed when the next line of code
is executed.

In contrast to this, as soon as I deactivate the hook_aggregator_process implementation, I can click the
'update items' link as often as I want, I never get multiple items. The module I wrote is never inserting
anything into the {aggregator_item} table, so I'm pretty sure it's not the culprit. But check for yourself:
http://npaste.de/p/oe/