Messenger of Stan is generic (it even says so on the identifier of the EC version) so you can have multiple copies in your territory at the same time.
Isaac is correct that you can only have one copy of a unique character in play at a time, that just doesn't apply to Messenger.
You can only have one copy of any Messenger in a T1 deck of 99 cards or less. You can have up to 4 copies of the same card in a T2 deck of 149 or less. Because if any two of the following are true, it is the same card for deck building purposes:
- Same name
- Same brigade
- Same art
Both the pale green and orange Messenger share the same name and art so they are considered the same card for deck building purposes.
