What IS a duplicate? [duplicate]

I've seen many challenges get marked as duplicates on this stack. Most I see as fair, some, not so much.

What qualifies as a duplicate question?

Consider challenges where:

• Entries from a will be valid for b , and entries from b will be valid for a
• Entries from a will be valid for b , but entries from b are not valid for a
• Entries from a will not be valid for b, but the challenges are similar in structure and rules.
• This qualifies as a duplicate. Nice example. :P Also, if you don't think the linked question completely answers your question, add a comment over there. There's no need for a 2nd question. – mbomb007 Dec 12 '16 at 17:04