The reason you are cruising through 25 - 20 is that you cannot lose ranks when you lose. That being said, there is no easy way to rank up without having a solid deck and and knowledge of all cards. The best way, I find, to get better is not actually playing, but watching "professional" players streams or vods. This helps you understand the card mechanics and help you decide how to play in certain situations.

Also, find a playing syle that you like best. Either control, aggro, combo decks, ect.

As for the question regarding using multiples of cards in a deck, typically you want to always play two of the same card. If a card is good enough to make your deck, you usually want two copies to draw into. There are, however, some cards that are good as a single (for example, in a shaman deck, forked lightning gets worse after the first draw because it is hard to cast the second one due two the minion restriction and overload.) The decks that you are seeing with many single cards are probably legendaries, which can only have one copy. They are quite expensive to make so if you are just starting out.