MacBook with Lenovo C-Dock duplicates image on external monitors
The MacBook only supports dual displays including the built in display.  In other words, it will only support one external monitor.  Per the tech specs:


  Dual display and video mirroring: simultaneously supports full native
  resolution on the built-in display and up to 4096-by-2304 resolution
  at 60Hz on an external display, both at millions of colors


The Lenovo C-Dock doesn't enable your MacBook to supply more display bandwidth than Apple provides.  So, the mirroring you are seeing is expected behavior. 

From the comments...


  ...I should specify that my MacBook is actually MacBook Pro (15-inch,
  2018). Would that make any difference?


Yes and no.  

Yes in that the MacBook Pro (as opposed to the MacBook) can support more than 1 external monitor.  However, as you've already discovered, if you connect them separately, it works.  Unfortunately, you can't daisy chain monitors from a single port; you must use separate ones.  Use of the dock still doesn't increase the available video bandwidth of the Thunderbolt/DisplayPort port even if the machine itself has the capability.