Cabal's --enable-library-coverage flag generates the .mix files in the dist/mix directory that cabal creates during compilation. I have two test suites, integration tests and unit tests. Those test suites calll tests for moduleA in moduleATest. Unfortunately, in a first version, the integration tests and unit tests for moduleA were both present in moduleATest. Therefore, the unit test suite (in UnitTests.hs) and the integration test suite (in IntegrationTests.hs) both 'use' the moduleATest by linking in tests from that module. This leads to cabal generating a directory integration-tests and a directory unit-tests with .mix files, but the moduleATest.mix file is present in both directories. When performing hpc sum and hpc markup to get the total result of the test runs I need to specify the directories where the mix files are present (so dist/hpc/mix/unit-tests and dist/hpc/mix/integration-tests) and it will then complain that it finds moduleATest.mix file twice.

This issue is not present when using the -fhpc flag, as the directory structure with integration-tests and unit-tests is not present in the .hpc directory at that moment. That being said, I don't think it's a cabal issue as it's more or less expected what they're doing, generating the mix files necessary to instrument integration-tests and unit-tests separately.

Priority put to lowest because I can work around it by diving moduleATest up in moduleATest and moduleAIntegrationTest, which is saner in the end anyway. But I guess somebody will come up with a use case where a split up isn't possible, in the future.