Stopovers in the same city on GLOB34
 

I've only done xONEx's and other alliances' RTWs, and multiple stopovers in the same city aren't allowed on those. But I can't see anything in the xGLOB34 rules that prohibit it. Am I missing something? If it's allowed, then JNB-HKG-NRT-BOS-MIA-BOS-SFO-BOS-LHR-JNB would be a valid itinerary, yes?

When rule changed ?
(I believe no rule changed)
XONEX have permited multiple stopovers in the same city since inception.
In ths past, even same city pairs can be flown multiple times in the same direction.

My old AONE3 included following Asian segments (with purcharsed extra segments)
-HKG-NRT-HKG-NRT-HKG-NRT-HKG-
Extra 1000USD(500USD x2) first class HKG-NRT-HKG was almost steal.

What Wasabi Tofu said is true

Yes, you are missing something
Your itinerary is valid (in particular, there is no restriction on the number of trans-continental flights like there is on the xONEx, and so your BOS-SFO-BOS segments are OK)
BUT the xGlobx restricts the number of stopovers in a region to four
I suspect you're wanting five in Nth America (BOS, MIA, BOS again, SFO, and BOS yet again)

If you change one of your trans-continental flights to have a connection (say SFO-DFW-BOS) then you'd have a valid xONE4 (and in general xONE4 tickets have the same base fare as the xGLOB34 from the same starting point).
And you could then make full use of the allowed 16 segments (no total mileage restriction on an xONEx): more stopovers in Asia at the start perhaps, or stopovers in Europe at the finish

This thread might help Musings: Circumstances where a Global Explorer might be better than a Oneworld Explorer