VNC Viewer appears multiple times in the Dock on Mac OS Mojave

This is caused by the new "Recent Apps" feature introduce by Apple in Mojave. Please follow the below steps to disable this behaviour:

1. Open System Preferences (Apple menu > System Preferences)
2. Click on Dock.
   
   
   
   
3. The last setting in the Dock preferences is a checkbox for Show recent applications in Dock. It’s checked by default. Clear the box to turn it off.
   
   
   
   
4. Close the preferences window.