Typechecking with multiAssignableTo.errors

[Dspil]

Typechecking with multiAssignableTo.errors using two times the same identifier in 'lefts' in a short var declaration passes typechecking when it shouldn't.

An example is the following:

for i, i := range x {...

which typechecks.

[jcp19]

This is related to a more general problem in Gobra where we often forgot to check for redeclarations, e.g., https://github.com/viperproject/gobra/issues/484