In a similar vein to #2322, dune utop hits a conflict when there are multiple implementations of the same virtual module in a repository. Perhaps the default_implementation could be used and the other virtual modules deselected from the toplevel?

$ dune utop
Error: Conflicting implementations for virtual library "io-page":
- "io-page-xen" in _build/default/lib/xen
- "io-page-unix" in _build/default/lib/unix

PR for io-page that shows this problem:
mirage/io-page#59