preamble-no-dup

# `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.)