This usually happens when a keyword or automation is being triggered again after the user sends a message. I’d recommend reviewing your keyword setup—there’s a good chance that one of the words in their reply is unintentionally triggering the auto DM flow again.
Also, something that has worked well for me is disabling the Default Reply feature (you can find it in Settings). This feature is triggered whenever the bot doesn’t recognize a message, which can sometimes restart or interfere with your automations.
By checking your keywords and turning off the Default Reply, you should be able to prevent the auto DM from being sent repeatedly.