Comment # 4 on bug 886906 from Lukas Ocilka

This is definitely the expected behavior. Control center can open more
different applications (here: Yast modules) at the same time.

Explanation: Similarly to an undefined desktop environment, you can always
open several windows at the same time with different applications. Use can
also open several control centers, so why not Yast modules? 

But we could rephrase this a bit...

Use case: I, as a user, always want to open a single instance of
          an application when launching it from control center, in order
          to prevent from changing the same configuration from two or more
          instances of the same tool.

Side note: Firefox let's you open the same page three times. Thunderbird
let's you connect to the same IMAP server from three different instances.
You can also run several xterms and the very same Yast module in it.

>From my POV, this should be done per module by locks: If there is a potential
of breaking configuration by modifying it using several instances of the same
Yast module twice, Yast should tell the user that a.) The same Yast module is
already running and tell them to close it b.) The same Yast module is already
running and whether they really want to continue (might make sense if the 
subsequent run is, e.g., for read-only access)

All in all, this is a feature request and if you really want it, I suggest
using https://features.opensuse.org/ for creating new feature request.