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.

This was working for me (15" MacBook Pro, 2018) with 2 Dell monitors connected using a ThinkPad Hybrid USB-C with USB-A Dock, with a single USB-C cable providing my laptop with power and driving the two external monitors as separate displays (not mirrored). Unfortunately that dock went back to work, and I bought a ThinkPad USB-C Dock Gen2, which looked similar. Using this dock, the external displays are mirrored. I have the DisplayPort software installed and I have tried everything I could think of. So I'm going to try and negotiate a return / replacement.