RNG is RNG... (I know that, 0 T10 loots in 10 clears).
For each item in T10, you have 10% to have it (items are chest dependent), 11.11% for an item on T11 and T12, 10% for T13 weapon and 14.29% for other T13 item.
Having 6 times the same item is really rare, but not impossible regarding the number of players.