We think specifically there may be issues when the same contact has multiple executions of the same email within rapid succession.
I am speculating here - but if Eloqua internally has a staging mechanism, and the same contact has multiple email executions in that same staging batch - it seems to cause an issue.
Even though all the inputs independently are validated.
@Lou Patrick did you get chance to see this
An SR should be submitted to thoroughly investigate the cause of the failure.