Typechecker: two behaviours with the same name should be rejected

[d-xo]

The following currently typechecks:

constructor of C
interface constructor(uint24 _x)

creates

  uint24 x := _x

behaviour increase of C
interface increase(uint24 new_x)

storage

  x => new_x

behaviour increase of C
interface increase2(uint24 new_x)

storage

  x => new_x + 1

But it should fail since we have two behaviours of the same name.