Heimerdinger turret stacking, bug or feature?

I was playing against a Heimerdinger and I took down his turret only to see the turret was still alive...

Then it happened again and I later saw him place a turret inside a turret, their health bars were one bar. as well as their charging bars. He got all 3 turrets into one exact area. Weird, I thought these objects had collision?

Needless to say this was extremely frustrating to deal with as an single target champion, it's not sour grapes, I think the Heimerdinger probably had my number without using this tactic anyway (and they had a better team gg)... But I have to say this seems a bit silly, I can't track the turrets charge properly - I can't track their hp properly or even know how many turrets are firing without careful examination of rate of fire.