Using the same record field name in different records:

https://downloads.haskell.org/~ghc/master/users-guide/8.0.1-...

Technically the complaint here was of the same record field name in different constructors for the same record, which is perfectly allowed when the fields in question have the same type but not when the types differ. I don't think DuplicateRecordFields addresses this.

That's correct.

    {-# LANGUAGE DuplicateRecordFields #-}
    data A = B { x :: Bool }
           | C { x :: Int }

    Example.hs:2:1: error:
      • Constructors B and C give different types for field ‘x’
      • In the data type declaration for ‘A’