I just bought beats but their serial number is already registered what do I do ?

Short Answer:

You don't need to register it. 

Long Boring Answer:
Normally when product documentation asks you to register a product for the warranty or whatever, it's mostly to just get your email so they can send you emails to improve their marketing. 

If it goes faulty, the retailer you originally purchased it from will have to honour the warranty provided you keep proof of purchase (receipt). 

Did you buy them used, or new? Did they appear to be a "returned" item?  

I think a lot of this remains questionable.  Are they authentic?  It might be that some group made up a bunch of fakes and use the serial number over and over again.

I've never had to register a pair of headphones, part of the Beats marketing doubling dupe I think...

Provided you have the receipt you'll be right. 

What did you learn from this experience? Hopefully it’s to stay away from Beats… (I’m just kidding…sort of…) Don’t know where you live but where I’m from, fakes are readily available at local flea markets. They sell them for $50-$75 and a lot of our high school kids around town have them. I teach at one of the high schools and was asked if I wanted a pair. I told the student that I wanted to hear them first. When they brought them to me, the box looks legit and then you take out the headphones and they are almost up to par. Sound was atrocious, even worse than the real Beats…What surprised me was I scanned the upc on the box, with my smartphone it was able to find them and pick up the barcode. All in all, like wje said, I would not be surprised if this serial # has been used by some counterfeiters. If you got them from a reputable dealer, and have your receipt it’s your call if you want to return them.