This happens a lot when the system is bogged down with traffic.
Like New Years Night was super horrible, I sent a few texts saying happy new year to a few folks, and two called me reporting they received the message atleast 4-5 times over the course of an hour. Then around 4 AM I received no less than 100 messages all at once from multiple people flooding in - some of the messages repeated upto 10 times, and it was obvious they were sent at Midnight or a little later.
It has happened on a smaller scale in this area too like when our schools had it's graduation, and other times traffic is higher than normal. There's a delay in sending/receiving and sometimes the server sends the message multiple times when it failed to send the first time.
NOTE: It isn't a service issue, it is just a problem it seems with the SMS service getting bogged down like I said. During these times I can make/receive calls, get on the internet, everything fine.
My opinions are my own and do not reflect the opinions of my employer AT&T.