I inject weekly because of the hormonal fluctuations. Even doing it once a week, I can tell the difference between the troughs and the peaks. I have been on 10 for 10 months, so I'm going through the big changes right now.
There's also the issue of amount. You are injecting 2x(ish) as much at a time, which is harder on the area, both volume-wise and having to soak it up and process it all.