At the lower ranks, double promoting isn't really a big deal IMO. In my previous school, I double promoted twice in color belts. I learned the required curriculum from my Sensei for the next rank before the test. Why did he teach me it? I was in class 4-5 nights per week, usually taking class twice each time (being a single college student, I had some down time). He taught me higher rank stuff when I was ready to learn it, not when some arbitrary time schedule demanded it.
I've double promoted twice in my current school. I know most of the curriculum up to 2nd dan due to my previous school being an offshoot of my current one. My current teacher's mentality is why hold me back if I've demonstrated the required skill level.
He asked me how I felt about double promoting a 3rd time, and I told him it doesn't matter to me. I'm not in any hurry to get anywhere (rank wise), and I'm improving, so what's the difference.
Keep in mind this has always been at the kyu level, not at dan level. But let's say I was a 5th dan is system A, and went to system B that was extremely close to system A, should a teacher be criticized for awarding me an equal or near equal rank if I proficienly demonstrated all necessary requirements for said rank?
IMO as long as the student meets all requirements for the rank, then there's no reason to hold the student back. So long as their skills aren't questionable, they've earned the rank. Give it to them and move on. Holding someone back solely due to a time schedule doesn't make much more sense than promoting someone solely due to a time schedule.