Samsung Nexus S turning off?

I bought a Samsung Nexus S on the Vodafone $29 cap and I've had some trouble with it turning off.
When I click the power button to sleep it the next time I click the power button to unsleep it, it is fully turned off. It is not possible to turn it off while holding it down for too long. This has happened many times and throughout the day it could happen up to 10 times. Should I take it to the Vodafone shop to ask for it to be repaired or is there any easy fix for this?
It has got the latest software so I should take it into Vodafone, they will probably take it and get a new one out the back or take it and send it to Samsung
 
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