Adding related issues are not de-duplicated

When adding Related issues to an issue, the related issues list should be de-duplicated. When adding an issue, it should check to see if that issue is already marked as being related. It should not be possible to add mark the same issue as being related multiple times. This just creates a mess and serves no purpose.

See for example the mess I created with these five inter-related issues. I assumed that a relation would only be added once and copied the same string of cross-connected issue numbers into all the issues. They ended up adding each-other to the list, making all the relations be duplicated. This should have ended up with only one set of relations, adding the same relation twice (even the other direction) should have been a NOOP.