You said " and this past Tuesday 08-27-2016 and they measured the fetus and it was 6w and 6d." Last Tuesday wasn't August 27. Did you mean this past Tuesday 9-27-2016?
If you meant September 27, then conception counts back to August 24, as the doctor's office said. This would suggest pregnancy by the first guy, not the second guy. Not by a huge margin, but an early ultrasound does not have such a large margin for error that you couldn't make this educated guess. It does mean you should do a DNA test when the baby comes, but I would go through pregnancy assuming the baby came from the sex from the 20th to 24th.
Thats what the nurse told me and this past Tuesday 08-27-2016 and they measured the fetus and it was 6w and 6d. When I first took my pregnancy test they said my conception date was 08-24-2016.
Is the only evidence you have of being 7w1d the original pregnancy test that said three weeks 6 days? I can't really get 7w1d if your period began on 8/24; counting from your last period you would be more like 9w1d. Did you see a doctor yet and get an ultrasound?