Steam support regarding duplicate CD keys fresh from retailer

Its your fault.

So what?

Steam is NOT responsible in cases like these. You have to talk to your retail shop for that.

As people have been said - Steam's not at fault here. Your retailer is.

Get back to your retailer and abolish them.

No one stole your key, whoever activated it long time ago isn't a thief. Your retailer is at fault for providing you with a used key, get back at them and use Steam Support's reply as a proof that it was an error on your retailer's end and it is their responsibility to send you a key replacement.

LPT:
Contact the developer.

Last time I contacted Steam Support they gave me 10 free games from the inconvenience. I was quite grateful for that.

why does steam take more than a week to tell me to go fuck myself and ask somewhere else
Well, simply because the size of the user base on Steam is significantly bigger than your retailer's, and therefore Valve is getting a lot more tickets, especially during summer sale.

I find it very stupid and dickish of you to call the one who has it activated on his account a "thief". How do you know it was not a factory error, or, if the box is as crappy as you say, how do you know the retailer did not simply use the same code twice?

Steam support is hit or miss. I would highly recommend going to the retailer. I'm sure they have better support. Also, logically if you bought something from a store physically or digitally and it does not work, wouldn't you go back to the store to get a replacement or refund? Steam didn't sell you the key or game, the retailer did. So Steam is in the right of course.