I have a similar situation with an Iphone4S. Went into vodafone and they told me I was due an upgrade so went along with it and walked out with a new phone. Phone had a fault so told them and they said they would send a new one out by post or I can collect from store.
Next day I collect one in the store, but keep getting texts to say my contract is due for renewal etc... Go into store to renew a different contract and walk out with another new iphone4S but upon inspection of the paperwork they have just renewed the same contract as they did just days before
I explain the situation, they don't really care and insist its correct, they dont want the phone back. I still have the other iphone4S in its box, has been there for months now.