[Haskell-cafe] stack: duplicate modules in library

I have come across a problem with haskell stack putting duplicate modules in a library. On "stack build" I get "Cabal-simple_mPHDZzAJ_3.0.1.0_ghc-8.8.4: Duplicate modules in library:..” This may have originally been caused when I changed capitalisation of a filename, but changing back did not resolve the problem. My question is how do I get rid of the library so I can rebuild? I tried "stack purge” and also removing ~/.stack with no luck (the latter causes a new ghc download on stack build, but with the same message afterwards) I know the source modules are fine because they all load in "stack ghci” Is there some other directory that stack/cabal uses for binary cache that I can safely delete? Chris