Steam FAIL or PayPal FAIL

This has happened to me before. If you put a ticket in with Steam they should be able to refund you. If they do not do this, dispute the payment with Paypal.

Do not dispute with paypal yet. As Crazee said, send in a steam ticket first. Remember it can take steam a while to respond (they really need a larger support team). Make sure you do tell them that you do want one copy of the DLC.

So if you have started a dispute with paypal, you need to stop it right now if you can. You might have issues with paypal and steam in the future if you don't, I have heard of others with issues when they started a dispute with paypal instead of trying to get it fixed in steam first.

dispute is close but i can't re-open again.
last time when i contact support i need to wait 3 day for response from them.

FYI closing a dispute automatically remove you from being able to do a claim. Don't open a dispute if it's not needed, and specially don't close a dispute if you really want to do a claim at some point (you will have to get a refund from the seller, paypal will not help you after the dispute is close).

I'm not sure about this situation though.

1. It's probably your browser that screwed you(It happened to me a while back) and it's surely not them(Nobody ever reported any serious problems like yours)
2. You should first contact them as they will help you a lot faster than Paypal
3. You will not get any refund back if you have already opened a dispute/claim thing on Paypal cause VALVe has a lot more reputation than you do so you will loose the money and your Steam Account

As people already mentioned, disputing your Steam purchases with Paypal can potentially make you very unhappy. Use the Steam support thing, they may be slow to respond but they aren't going to cheat you. Good luck! :)