Uniq the list of unknown extensions to avoid "Warning: Unknown extensions: TypeAbstractions, TypeAbstractions"

When building a package with `TypeAbstractions` default extension in .cabal and my cabal version is not aware of TypeAbstractions, I'm getting

```
Warning: Unknown extensions: TypeAbstractions, TypeAbstractions,
TypeAbstractions, TypeAbstractions, TypeAbstractions, TypeAbstractions,
TypeAbstractions, TypeAbstractions, TypeAbstractions, TypeAbstractions,
TypeAbstractions, TypeAbstractions
```

probably because the `TypeAbstractions` is in a common stanza, which is then used in a dozen components in that package. It would be great if cabal just said "Warning: Unknown extensions: TypeAbstractions". (To add insult to injury, this message is repeated for each of the dozen component, but that is fine and less confusing than reporting warnings for common stanzas separately from components).

I didn't mark this as a `newcomer-one-line-change`, because it needs a test, which would have several lines.