Allow duplicates generally. Disallow repeat (back to back same item) duplicates. After receiving the same exotic three times, that item is taken out of the pool entirely until all others have been collected at least once. This allows players to receive potential upgrades for the exotics that they already have, but not to the point where they feel punished by the system. After receiving the same item three times, the player;knowing that they will not receive that item again will at least feel like they are making some progress towards getting the few exotics that they likely hope for. Player retention is obviously important, and I believe that this method effectively tackles both sides of the issue. The "casual" side of the market will more or less be unaffected by these proposed changes and would continue as normal likely not noticing any difference. The players that grind until bored will feel rewarded for their efforts and be more inclined to keep playing. Casinos and "one armed bandit" machines have used this method for years. Every now and again the machine has to pay out or the player will leave. The current system doesn't pay out.