It's common in mac to drag the app into the dock to create a shortcut for it. In macOS Sierra 10.12 this functionality doesn't work as it should.
After dragging Blender into the dock, when clicking on the Blender Icon in the dock it opens the app in a new instance on the dock.
As you can see app is already in the dock and then when clicked on it opens a new icon over to the right.
Just a bit odd..