Well, I think one answer is to add another service in front of that shared business entity that exposes an interface to match the domain of the consumers. Now the shared codebase should be a stable client.

But yeah, of course there's shared code between your codebases - things like JSON libraries, etc, are always going to be shared. The issue is more about what you're sharing - if it's a very stable, small library, it's probably fine. If it's something really core that's going to change a lot, or that two services may diverge in agreement upon, duplication may be worthwhile.