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The DM is also free to assign advantage and disadvantage as they see fit. So maybe rather than having Urkel auto fail, I give Urkel disadvantage on the check or Johnson advantage, or both. At that point, while some possibility may exist of Urkel beating Johnson, it will be fairly miniscule to be certain.
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Agree with everything else in your post except this
This is a bad example for some reasons: It is a contest, and armwrestling is much about technique so it is not a situation where I would disallow a contested roll. To give the muscle man advantage would be a bit unfair except he had a feat saying to do so. And it would not prevent the possible (although unlikely) outcome that he loses.
Better example would be Johnson and Urkel decide to lift a heavy weight. And here I would say Johnso has the chance to beat e.g. DC15 alone and gets advantage if Urkle decides to help him. Urkel alone might not try but if it only were some kilograms so the DC is 5 then he may try as well.
That is consistent, as demanded by @Saelorn