I just looked at a house with a 200 amp 1-Ø service. The contractor installed a #4 solid copper conductor to the rebar in the foundation. He attached the conductor to the rebar with an acorn clamp (listed for use on 5/8" ground rod.) He then attached another #4 solid copper conductor to the rebar under the same clamp and connected it to the copper water pipe.
I believe this installation is in violation of the NEC as the ground wire should be continous. I also think the connection to the rebar should be an exothermic weld (Cadweld).
What do you think?

I think the dude cheesed it out, a cadweld would be an improvement, and as it may be undetermined at to the rebar's integrity,( cheesy Ufer too?), a few G-rods would not hurt.

The grounding electrode conductor (GEC) has to be continuous to any available electrode.
From that point the GEC stops and a connection to any other electrode is a bonding jumper. The connection to the rebar is OK if the acorn clamp is listed for direct burial/rebar/concrete encasement.

But two conductors under one clamp is most likely a violation of 110.3(B)
While a cadweld connection may be desirable, Burndy would like you to know they make a compression connection they belive is superior. Whatever is used must be listed for the application.