The idea when it comes to collecting is "try to get as many as you can...within reason". This goes for any type of collection: stamps, coins, etc. Obviously if you get only one auto/coin/stamp you will still be very thankful to add it, but if you are able to get more, why not try? This is where "graphing etiquette" comes into play, but you can see my point. Duplicates can be used to trade and expand your collection. Scoreset has helped me a ton personally + gave me amazing deals with his duplicates.