Reproducible examples of an issue key that identifies more than one issue?

[Keiei Tanto [Vivid]]

It seems possible for an issue key to identify more than one issue simultaneously, and vice versa. In what circumstances is this possible and how can those circumstances be reproduced? I want to see this in action myself.

Two approaches I can offer:

• Renaming an existing project (ID = 10005) key from ABC to DEF, and then creating a new project (ID = 10073) with (overlapping) project key ABC. Issue #1 from the former project #10005 is then identifiable with issue keys DEF-1 and ABC-1. Issue #1 in the latter, newly-created project #10073 is mapped to ABC-1. In this situation, ABC-1 refers simultaneously to two issues: issue #1 in project #10005, and another issue #1 in project #10073.
• Project keys are alphabet-wise identical, but actually differ by case. For example, there are two projects, with keys "xyz" and "XYZ". Using JIRA's Quick Search to search for "xyz-1" or "XYZ-1" returns both issues in either case, keyed by "XYZ-1" and "xyz-1".

For example, these API calls have signatures and implementations that indicate the premise is indeed possible, such as:

• https://docs.atlassian.com/jira/latest/com/atlassian/jira/issue/IssueManager.html#getAllIssueKeys(java.lang.Long) (JIRA 5.1)
• https://developer.atlassian.com/static/javadoc/jira/reference/com/atlassian/jira/jql/util/JqlIssueSupport.html#getIssues(java.lang.String) (JIRA 6.3.15)

Also, what is best practice in handling such issues?

I ask in the context of an add-on developer targeting JIRA version 5.1 and newer + systems whose data has been upgraded from older versions where rules may have differed. This topic affects developer awareness, vendor's customer education & customer support activities, and JIRA plugin technicalities.

[Nic Brough -Adaptavist-]

Yes, lots, almost every JIRA in the world is using "issue links".  See https://confluence.atlassian.com/display/JIRA/Linking+Issues

[Keiei Tanto [Vivid]]

Your reply serendipitously provides validation to my situation. But I chose the wrong wording -- sorry about that. Updated question.