Why does my IPhone randomly play my message tone?

It's an iPhone 4, I've had it for around 3 years and its started not so long and doesn't happen everyday or week even, just occasionally
I'm not sure if it does when I'm on it or just at anytime, I mostly notice it when I'm on it, but I believe it might of happened when I was not using it
Is it broken? Does this happen to other people iPhones?

This may be due to the fact that you have your notifications set to repeat. Whenever you get a text message, the default setting is that the alert will repeat once. To change this, go to:

Settings < Notifications < Messages < "Repeat Alert" and you can change it to "Never"

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