parsemail: Catch errors caused by duplicate patches in revision

Let's assume mail structure with replies like this:

[1/2] Patch 1
  |- [2/2] Patch 2
  |- [2/2] Another Patch 2

When the "Another Patch 2" comes in, it's going to get inserted in the
same revision at the same place as the original "Patch 2", which results
in integrity error.

This is the single most common reason for administration notifications.

There is not really much we can do, as the reasons for such emails are
so ambiguous that the best we can do is to discard them, just like we
are doing now. So let's make it silent.

Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
4 jobs for master in 7 minutes and 11 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Containers1
passed #113412
containers1:patchwork-base

00:02:08

 
  Containers2
passed #113413
containers2:patchwork-testing

00:02:19

 
  Test
passed #113414
test

00:01:28

 
  Deploy
passed #113415
deploy:patchwork-deploy

00:01:15