The Mummy paradox of which you talk tells you that that card could be unbalanced.
Mummy costs 4
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and is a 5/3 creature + an ability with a duo requirement. So that adds
4
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= 5(attack) + 0(HP=3) + 1(Ability) - 1 (Requirement) - 1(Duo)
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This seems to be a hole in the calculations. If we gave Abomination, or any other vanilla creature, an ability with a duo requirement its cost lowers by 1.
LOL
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I didn't notice that. NICE
OH! and upgraded versions seems to always cost -1 or -2 of what they should cost.