Good call making this a common, I could see some people feeling pretty cheated if it were harder to obtain. In fact, I think the worst case scenario would be having it max rarity with 1 favor cost to unlock 4 slots, so you are far enough away from that to probably stifle most negative reactions, but there still might be a problem to be found here.

In terms of the balance itself, I think 3 for 3 is good enough and also intuitive to think about when considering to use it. Although 2 for 2 might be a less severe initial negative impact: please carefully consider that for some people this change may completely disable all of their armies/strategies, and require coins to remedy. This problem exists independent of balance, so the change tilting towards the extreme side looks hazardous to me.

Anyway this topic also brings up the point of duplication, which if I recall correctly is a unique that changes limitation from 2 copies into unlimited. I think it might be more interesting to have duplication add some flat value, but at reduced cost, and where the final value from max duplication is again effectively unlimited. This would make it seem more reasonable to use a single duplication for things like "I just want one more of those" instead of "my entire army is this unit" because with the single copy of duplication allowing unlimited it may always be balanced around that use case, rather than having value relative to the advantage of using 1 or 2 extra.