Redmine

When mentioning another issue in the description or a comment with hashtag, this could automatically trigger the setting of a relation to this issue. The type of relation could be predefined in the project settings.

For example:
Someone opens a new issue for reporting a bug.
The developer comments something like "That's the same issue like in #1234"
With posting his comment, redmine recognizes the hashtag and the issue ID behind it, and sets #1234 as "Related to" in the current issue at the same time posting it.