Haddock options are duplicated

When I generate documentation with `cabal new-haddock --haddock-options "--qual local"` I see the following error:

```
/opt/ghc/bin/haddock [...] --qual local --qual local
haddock: qualification option given multiple times
```

I guess haddock could be a bit more relaxed about repeated options like this. Nonetheless, `cabal` should be fixed.

Start by reproducing the `cabal new-haddock --haddock-options "--qual local"` command and trace the `new-haddock` option handling. Check where the supplied Haddock options are combined with generated options. Done means each option is passed once and the command no longer reports a repeated qualification option.

Written by the indexing model from the issue text.