I've never measured the current flow from a working alternator, but if its supplying 10A to the battery, where say 8A is leaving the battery to drive the active loads (lights, ignition, etc.) and the other 2A is actually charging the battery, that would seem reasonable. I would assume that an excessive load that could burn out an alternator that fast would blow a fuse first, but anything's possible. Hard to say if the identical failure mode is due to an identical improper rebuild, or something wrong with your electrical system. The fact that your original alternator went out might suggest there's something overloading your electrical system that caused the initial failure, but not necessarily. If your measurement actually indicated that 10A was charging the battery, in addition to driving the other ~8A or so of load, then that is probably not normal. A 14Ah battery like most motorcycles use typically needs 1.5-2A of charging current.
I'd try to measure the current load from the various systems with the ignition on and the bike not running, and see if anything abnormal shows up. See if you can find a small resistive load that you can measure for voltage drop to calculate the current flow (like a fuse or section of large gauge wire). Then start it up and measure the load again. Sorry I can't be of more help, perhaps one of the folks who have rebuilt alternators can provide more info.
Edit:
The fact that you measure 14.2V at the battery while under load is normal. If you have a very high current draw you usually create a larger than normal voltage drop across the ignition switch, fuses, etc. and the alternator senses that drop and raises the battery voltage to compensate. Or a corroded connection provides a false high voltage drop to the alternator and it bumps up the voltage accordingly, which usually overcharges the battery in addition to taxing the alternator unnecessarily. The fact that you aren't seeing that may suggest that your system is normal and your rebuilt alternators were not done to spec. How much do you know about the guy rebuilding them, has anybody else you know used him successfully?