I have been observing strange behavior with two dictionary apps from a French / Belgian developer. I can launch both of them multiple times -- i.e., multiple instances of the app will appear in the dock (and Activity Monitor) -- but this is true only for these apps. I'm running Mac OS 10.13.3 on a MBP mid-2015. I've reported this to the developer, but they can't reproduce the problem -- and state that no one else has reported this. If I create a test account, the problem still occurs. I've tried completely deleting and reinstalling these apps, to no avail. However, it turns out that I'm unable to launch more than one instance of these apps on a 2010 MacBook Pro running Mac OS 10.11, and I don't know whether it's because of the hardware or the software.
Does anyone have any suggestions as to how to troubleshoot, if not how to resolve this problem?