[Haskell-cafe] stack: duplicate modules in library

[Chris Reade]

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