Gas vents can auto build connection without dupe building

if you have two pipe tiles adjacent to each other but not connected, connecting them just requires a click-drag-release action with the right building type selected, no material consumed, no dupe engaged. This logic is consistent for gas pipes, liquid pipes, wires and transport bands. I'd say, not a bug, a feature.

The idea is that you already paid for the material in both tiles, so no new material is needed.

However, I still agree with you that this is an exploit and if you're making a change in the piping a dupe should come to finish it. Making an instant change is not right(although might help).

I agree with no materials needed, but it's inconsistent behavior that it instant builds.  It screws up your builds if some items place blueprints while others instant build then you have deconstruct the 2 tiles to break the connection and rebuild them with a dupe.

Yes it would be nice is plumbers or electricians where appropriate could do a task that without deconstruction disconnects a pipe or wire from every adjacent tile so you can reset it, these same dupes could handle making new connections with adjacent tiles with kind of a just get a wrench out and fix it animation.