When creating a new Issue Link Type, you need to specify an Outward Link Description (e.g. "duplicates") and an Inward Link Description (e.g. "is duplicated by").
What do these mean?
When a JIRA user links two issues together,
- the Outward Link Description applies to the issue from within which they clicked "Link this issue to another issue".
- the Inward Link Description applies to the issue that they choose to link to.