Why I can't register?

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I'm newbie in online bussiness. I try to register some ptc, but somehow they rejected me. They said : You're IP is already being used so you're not permitted to signup again. It also happen with Neobux. I already join that site, but when I clicked the add, it shown : Another user using your IP address already. Viewed this advertisement in the last 24 hours... How come?? While it was my first login after few days?? Is someone really use my IP? I try to contact a man who referring me, but don't get the answered yet.