Object placement and instancing

My (limited) understanding is that if you have a mesh object in engine and you duplicate it, it sort of “auto-instances” it so the duplicate is not as expensive as the original mesh. (correct me if I’m wrong.)
Individual loaded meshes are therefor more expensive.

But let’s say I’m working on a wall around a village for instance, and each piece of the wall is the same. If I’ve carefully laid those out in my 3D app, is there a way to load them in position to the engine, then say something like “make all of these objects instances of the original?”

Any clarification helpful!