Me and my friend were actually discussing this EXACT theory the other day. It all started because we began quickly realising over time, that when teaming up to do Nightfalls and Raids I would constantly get the same drops all the time, of gear and weapons that I ALREADY had, but they would be stuff that HE really needed. And vice versa, he kept getting stuff that I needed, but that he already had. It may just be coincidence, but there is definitely enough evidence there to suggest that the whole loot system is not exactly what it seems...