Do not allow to queue the same message twice

Add a new error to indicate the message is already in the session queue.
7 jobs for master in 4 minutes and 55 seconds (queued for 1 second)
Status Name Job ID Coverage
  Build
passed fedora-asan #1343228
asan

00:01:23

passed fedora-autobahn-quick #1343226
ipv6

00:04:55

passed fedora-scan #1343227
ipv6

00:01:34

passed fedora-test #1343225
ipv6

00:01:31

83.9%
 
  Docs
passed reference #1343229

00:01:07

 
  Deploy
passed pages #1343230

00:00:16

passed pages:deploy

00:00:01