The more I look at Houdini Engine the more I realize all created geometry is unique. Your tool automatically places 20 lamp posts along a street and each is unique with nothing actually reused.

How do you tell the system to place a copy of an existing piece of geometry instead of creating new?

Please have a look at the attached asset. But basically you need to create an instance obj node which will generate points (in instance tab, select fast instancing and point to another obj node which will be instanced). That's the gist of it. On Unreal side, this will create instanced static mesh component and use each point transform for new instances.