Problem about superposition theorem
You are misapplying the superposition theorem. When you calculate the response to a source, other sources must be passivated. In the case of a voltage source, the same is replaced by a short circuit.
If in this particular case, try to apply the theorem, both sources will be connected in short, implying that the voltage on the resistance would be zero, while each voltage source should deliver an infinity current.
Clearly, there is a singularity for which circuit theory of lumped parameters does not apply.

If the voltage sources they were replaced by real models, considering its internal resistance







simulate this circuit &ndash; Schematic created using CircuitLab

you can apply the Superposition Theorem smoothly.