I made object
A where the instances will go, parented object
B to it, and activated
Instancing on Verts (under Properties > Object Properties).
The instances appear as desired. Then I did 'make instances real' (I get to that by searching). Then I chose all the created instances, and joined them (Ctrl + J). Call that object
When I do that, object B becomes the same as object
C. It duplicates itself the same way. In fact, if I then go in and edit object
B changes in the same way. And vice versa.
For me, this is the best way to do this. It allows me to export the combined object as a glTF and also keep the template object for later reuse or modification.