hadrian: Throw error on duplicate-named flavours
hadrian: Throw error on duplicate-named flavours

Throw an error if the user requests a flavour for which there is more
than one match.

Fixes #17156.

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc