`preamble-no-dup`

There are no duplicate headers.

Examples

error[preamble-no-dup]: preamble header `header` defined multiple times
  |
2 | header: value0
  | -------------- info: first defined here
  |
4 | header: value1
  | ^^^^^^^^^^^^^^ redefined here
  |

Explanation

preamble-no-dup makes sure there are no repeated fields in the front matter (or preamble.)