Feature request: `cabal check` warn about duplicate entries in `default-extensions` and `other-extensions`

From: https://github.com/haskell-hvr/cassava/commit/3f792495c1256dc2e3a503b4c5c4518dfce9c5d8#r141267172

Suggestions to improve `cabal check`:
- warn about duplicates in `default-extensions` and `other-extensions`
- warn if `other-extensions` repeats extensions from `default-extensions`

In the presence of conditionals, this should not trigger false alarms though.
Only warn about real-existing duplicates!

In the wild: https://github.com/haskell-hvr/cassava/blob/2da75c97d6b3a27675cc90fa30bc55a88d0f3da5/cassava.cabal#L67-L84
```
other-extensions:
BangPatterns
CPP
DataKinds
DataKinds
DefaultSignatures
DeriveFunctor
FlexibleContexts
FlexibleInstances
KindSignatures
MultiParamTypeClasses
OverloadedStrings
PolyKinds
PolyKinds
Rank2Types
ScopedTypeVariables
TypeOperators
UndecidableInstances
```