Summary

At the moment, it's possible to create both an inward and an outward link of the same type (eg, relates to, is related to) to a single issue. This should be allowed to be restricted

Suggestion

Give an option to restrict having the same link (inward and outward) to the same issue

Use case

Sometimes the links will be named is parent of and is child of, depending on the organizations need. It would then not make sense for issue A to be a parent of issue B, and at the same time is also a child of issue B