Hey I want to buy set from 0.5 to 3 but I wanted to double up a couple of sizes. Which ones should I double the best for the first time? .75 to 2?

James has great advice, and I'd second everything he said, except for the part about the Link Cams. I would steer you away from OP Link cams (as a beginner), because they are much more difficult to evaluate than C4's, plus they're heavy and expensive to boot. Here's a straight answer: If i wasn't trying to supplement a friend's rack, and had to choose three C4's to own in addition to this set, I would get either (.4, .75, 1) or (.4, 1, 2), assuming you also end up buying some Wallnuts

Bottom line Mike, every climb is different, and some will require more of the smaller cams while others need larger ones. As a climber and someone who has many climbing friends, we all own and use the 0.5-3 sizes pretty regularly, with the occasional micro-cam or larger cam thrown into the mix. It doesnt really matter what sizes you double up on, because sooner or later, each size will find its use. If you have the time, check out what your climbing friends have more of, or see what is recommended for the climbs/crags in your area. You can also check out the Omega Pacific Link Cam, which has the size range of three cams (NEATO!) Otherwise, mix and match and just have fun with awesome gear. http://www.backcountry.com/omega-pacific-link-cam

Thanks You for reply! Thats right, Im collecting my first rack, but i wanted to double some sizes 2- 3 sizes on the start since I have got money right now and in my country cams are VERY expensive so I wanted take this opportunity to buy them cheapper. Maybe green and purple You think? Or it doesn't really matter which ones I'll double right now.

Mike, If your starting your own gear rack, and you have friends with plenty of gear to share, a single set of these is nice (0.5-3). It tells your friends you want to help, without making a bold statement. Eventually though, you will find yourself doubling up on all these main cam sizes, then the micro cams, then the monster cams. Cams are the meat and potatoes of any gear rack, and they hardly ever go to waste on most climbs. As with all tech gear, be sure you understand their proper use and placement. Climb safe.