How can I link duplicated issues?

Lets say I have issue: #1 and issue #4, and they are duplicated.
Is there a way to link them both so, if I close one, it will close both?

Nope, you cannot do that.

With the upcoming GitLab 9.4 (June 22nd), there’s a new feature in Enterprise Edition Starter It doesn’t quite do what you ask, but there may be a use case there that’s worth making a feature proposal

Thank you!!