New rule: Detect duplicate sections

[jfmengels]

What the rule should do:

Report duplicate sections in README and module documentations.

What problems does it solve:

Duplicate sections

Example of things the rule would report:

{-|
# section

# section

## init

@docs init

-}

{-| this is init -}
init = 1

Example of things the rule would not report:

{-|
# section

## Initialization

@docs init

-}

{-| this is init -}
init = 1

When (not) to enable this rule:

If you are working on an application and it has no documentation?

I am looking for:

• Someone to implement it (I can provide help and guidance)
• Question: Do you think this should be merged with another rule? If so, which one?