Is it possible for my email to get journaled more then once by a single hub transport server?
Yes it is. With Exchange 2010, it's possible to have that message journalized several times because of the way the transport expands the Distribution List. The transport will try to send the message as soon as possible, so it "chips" the DL into chucks of 1000processes each chuck with any enabled transport agents (including the journal agent) then sends the first chuck of 1000. If you have a DL of, say, 4500 members, you'd get 5 messages in the journal. You can change the size of the "chip" to a value large enough that no DL would have more than the value of the "ExpansionSizeLimit". You have to make that change in the file edgetransport.exe.config on each HT server and then restart the transport service.
This is explained on the following link:
If an email passes through multiple Hub Transport servers is it journaled more then once as it's being processed by multiple transport agents? Hub Transport servers marked as Hub Sites can cause emails to be processed by multiple Hub Transport servers along the way.
No emails are only ever processed by one journal agent. Once an email passes through the first Hub Transport server the Transport Agent or Journal Agent "stamps" the email as being processed in the message header with X-MS-Exchange-Organization-Processed-By-Journaling x-header, so other Hub Transport servers can determine it’s already journaled. This prevents the Journal Agent or Transport Agent from processing it more then once.