RBS::InstanceVariableDuplicationError happens on double included module

rbs validate shows RBS::InstanceVariableDuplicationError if one class includes a module twice. Is it intentional?

Problem

module M
  @iv: untyped
end

class C
  include M
  include M
end

$ rbs -I. validate
E, [2025-04-07T15:08:43.141421 #23447] ERROR -- rbs: ./test.rbs:2:2...2:14: Duplicated instance variable name `@iv` in `::M` (RBS::InstanceVariableDuplicationError)

    @iv: untyped

Real world example

I realized this problem in the following code in a real Rails application.

module M
  @iv: untyped
end

module X
  include M
end

class C
  include M
  include X
end

Our code base (unfortunately) has such code. The module M is included twice, directly and indirectly.

I guess it can happen in other cases on complex inheritance trees.

Also, the duplication is not harmful. Because these ivar definitions are the same. So I think we can suppress the error in this case.

Even if it should be an error, I think we should fix the error message because it's confusing.

Start by reproducing the two RBS examples with rbs validate, then trace the validation logic that processes repeated module inclusion and instance-variable definitions. Done means the behavior for harmless duplicate inclusion is decided and the resulting validation behavior or error message is covered by tests.