Do I have to pay for minecraft twice?

You can download Minecraft for free again at - https://minecraft.net/...craft.net/

Then when you have it downloaded, sign into your Minecraft account (that you purchased Minecraft on) and you're good to go play online!

No just download minecraft again and log in to the launcher and you should be good to go.

The IGN (your minecraft name you use for getting into a server/single player) goes with you and isn't linked through operating systems so you'll be able to use the Minecraft Username for any operating system that allows minecraft to be played on it.

The program you need to download to install minecraft to play it is free. But if you didn't basically buy a name to use within it, you'll need to buy that. But the minecraft client file is free.

www.minecraft.net to download the new client for your new operating system. Download it, install it, and it'll prompt you to put in the email address/password you use to sign into the client.

Yes you do. When you purchase a game, one o the things you are purchasing along with the actual product is the license to play the game. The intent is that only one person is allowed to play the game, and everyone who plays it should technically be buying their own copy of it. This is hard to enforce, so the general acceptance is that each "device" that you put the game on requires it's own purchased copy.

In your case, you chose a mac version of the game, and that license will not work for the PC version. You can try to install the game, and then unlock it with your code (if you purchased a digital copy), it may work, but I wouldn't expect it to. If you purchased a copy from a store, then your PC won't even be able to properly read the Mac version of the disc, unless both PC and MAC editions are on the same disc (which many companies do).