iPhone 5 says texts not delivered...but they are.

Hey all,

I just got a 16gb iPhone through Sprint. I'd say a quarter of the texts I send say "not delivered", but people text me back confirming they received them. So I'm sending a lot of duplicate texts. The texts I've sent have been to an iPhone (3g) user. I have iMessage, SMS, MMS all turned on and my send & receive number is correct.

Anyone else encountering this?