A card being auto-included doesn't mean it's too strong. It does make decks more boring, but that's no immediate reason to nerf the card.
Let's compare it with other factions (in a vacuum, I know):
Wild Hunt Rider
12 provisions for 8 strength
efficiency: 66,7%
Blue Stripes Commando
12 provisions for 8 strength
efficiency: 66,7% (but has orders)
Mahakam Volunteers
10 provisions for 6 strength
efficiency: 60% (requires a dwarf, but also synergies with other dwarfs)
Skald & Skirmisher (actually not a duo card)
9 provisions for 6 strength
efficiency: 66,7% (requires both cards)
Drummond Shieldmaiden
12 provisions for 6 strength (8 strength - 1 for the damage and -1 for not hitting an enemy)
efficiency: 50%
Impera Brigade
12 provisions for 6 strength
efficiency: 50% (but doesn't have a condition)
Witcher Trio
21 provisions for 9 strength
efficiency: 42,9% (but thins two cards, instead of one)
Looking at the above list, Wild Hunt Rider isn't too exceptional, even though it's the best one.
EDIT: added the other factions too. Every faction has a "duo" card that summon a copy from the deck. Some have better synergies with their deck, while others can be triggered more easily.
That is a really weak way of counting provisions.
Basicly all decks run exactly 25 cards, so you can reduce base provisions by 25*4 (as 4 is the lowest you have to include anyways) and reduce the provision cost of everything by 4, so provison costs range from 0 to 11.
That is a much easier way to look at things and that transformation holds true, as long as you play a 25 card deck.
Wild Hunt Riders are 6-4 = 2 provisions, so you run 2 x 2 provision to get an 8 point swing.
Witchers are 3*(7-4) = 9 provisions for a 9 point swing.
Gernichora + Griffin is 1 Gernichora turn for 7-4 = 3 provisions for an 8 point swing.
Slave Infantriy is a 7-4= 3 provision play to get a 10-x value play (x being the target's power), so 9 for a 1 point target.
Blue Stripes Commando is the same payoff, while not being dead if you do not have the strongest creature, yet require to survive a turn, which is (more often than not) problematic (at least if an opponent (like me) runs Ifrit, Chironex etc).
Mahakam Volunteers is a 2*(5-4) = 2 provision 6 point play, which thins and has next to no conditions to be used (difficult to brick), while having an advantage by being lower base provisions, being the same thinning, thinning 2 of the second lowest provision costs to get an alright power play.
Impera Brigade is a 2*(6-4) = 4 provision 6 point play that is about always droppable, which is more versatile than the others in that regard.
Drummond Shield Maidens do not really count, due to being unplayable trash (at least I cannot think of any proper way right now).
To be fair, aside from the Queensguards, Wild Hunt Riders and Impera Brigade I would not run the other Duo cards (outside of specific decks like the Dwarf, which is actually underestimated under this topic).
Though I agree on it being not too exceptional, too be fair I would even call the Dwarven Duo better, although Dwarf decks are in general horrible, so that does not say a lot.
And to be fair I am already considering to just skip Riders, just like I skipped Witchers in Monsters, the payoff for Riders is not as large as people assume, not to mention that noone can pass Round 1 before turn 3, which means tempo is not that much of an argument, especially when everyone has access to Unicorn and Chironex.
Given the position in this debate I am surprised people are not complaining about them anymore, given that they are still disruption, + 20 power for 18 provisions, auto include in about everything and a lot of utility.