Just in case anyone would have the same issue, I don't know about Maya 2019 but in Maya 2020 if you select only the nodes you want to duplicate in the node editor (I also had a bunch of math nodes to duplicate), you can select them and do the same manipulation, without having to duplicate the mesh they are connected to. First ensure that your node editor allows new nodes to be automatically add to your graph then Edit > Duplicate Special with "Duplicate input connections" and you will see your group of node appear with all the connections.
Another tip, before doing so, pin all your nodes, then disable the option "pin all nodes by default" so your new nodes will be nicely placed, otherwise they may all be at the same place, making you lose a lot of time to replace everything clearly.
Hope this may help someone.