1. On a metal spot, build a T1 mex. 2. Then build a T2 mex over it. 3. With a fabber near this metal spot, click-drag to build T1 mexes over the whole planet. A *second* T1 mex will be built on this spot, clearly displaying a progress indicator, and the metal income moves up accordingly. See replay 208003963282171102 for an example against the AI.
Yeah, I've seen this before. It's been around for a couple of builds now, and no fix has been issued yet.
I discovered this recently watching a replay. I saw the AI managed to put two T1 mex and one T2 in one spot. Then I managed to reproduce it in the latest pte myself. I think it is 100% reproduceable (if that is an actual english word).
On a hunch, I checked - this is actually an INFINITE mexploit. You can alternate placing T1 mexes and T2 mexes on the same spot, forever (again, using click-drag for a whole-planet build; direct clicks don't work, obviously). See replay 9965277911446433765 for several cycles of this.