In this case, a single mysql instance with individual databases may indeed be the best approach. It'd be very easy to launch a mysql container and have each wordpress container talk to it. I use Rancher for orchestration, and it automatically assigns an addressable hostname to each container/service, so I'd just pass that to each wordpress container. Or you could expose a port and use a normal IP + port.

The duplication is preferred because you can take that stack and launch it on any machine with Docker with one command. Database and all. Usually that's great, but it'd be very inefficient in this case.