Add new Lint/DuplicateElsifCondition cop

[fatkodima]

This cop checks for duplicated conditions in elsifs. This is almost definitely a programmer mistake due to copy-paste, refactoring, etc.

# bad
if x == 1
  do_something
elsif x == 1
  do_something_else
end

# good
if x == 1
  do_something
elsif x == 2
  do_something_else
end

This cop is in addition to existing Lint/DuplicateCaseCondition.

[bbatsov]

Seems to me this should be a Lint cop.

[bbatsov]

Seems it's a Style cop only in the PR description and the commit message. :-)

[fatkodima]

Haha, yes, it is a bad copy-paste 😄

[bbatsov]

Thanks!