Being that isn't not unique, If I have a copy in play and an opponent has a copy in play do all standing characters get +6 or just +3 since they're both the same card.

If it's +6 does it matter who owns them? ie I have two copies of King's Pavillion in play so all characters get +6 while standing.

Yes, it stacks and, yes, it applies to all standing characters, regardless of who is controlling them. If you're both running it, then odds are you've both built decks with that effect in mind... could be some crazy battle math going on there. :-)

cool thanks for the quick response,

I was actually trying to build a non kneeling army deck and thought that it would be a good inclusion, just didn't know if they stacked or not.

It can be crazy awesome if you've built the deck with that in mind. I once used multiple copies in a Baratheon The Wall deck (where when it's Winter, Bara characters don't kneel to defend) using high initiative plots (choosing to go second). It was a lot of fun. Not Tier 1 by any means, but there was much fun had with that deck.