When running R CMD INSTALL for the DirichletMultinomial package on my machine, I'm getting the following error:

Error: (converted from warning) replacing previous import ‘stats::mad’ by ‘BiocGenerics::mad’ when loading ‘IRanges’

After searching online, I see this has something to do with the NAMESPACE importing stats and BiocGenerics, both with the method mad.

Any ideas on how to fix this?

Following Michael's suggestion, it seems likely that you have a mix of Bioconductor packages from different Bioconductor releases. Use BiocInstaller::biocVersion() to get the version of Bioconductor in use, and BiocInstaller::biocValid() to identify packages that are out-of-sync.