WA message being sent several times during the day without me knowing.

Hi, yesterday I was sending a WA message right before my phone runned out of battery. The thing is that as soon as I sent the message, the phone went off but I wasn't sure the message went through. Since then, my friend (to who I sent the message to), has been receiving the same message every once in a while without me knowing. It's very weird and annoying. I checked my conversation on WA with her and the funny thing is that the message doesn't appear on it, apparently it is just floating in the air and hitting my friend's phone every once in a while. Does anyone have an idea on what we can do to prevent this?