ManyChat
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If you don’t remember setting up those messages in ManyChat - please, make sure to check Facebook Assistant in your Fb page’s Settings > Messaging > During a Messenger Conversation > Set Up.

Also, if you have any other apps connected to the Facebook page - the messages might be coming from those, so make sure to check the setup there.

If the problematic messages were set up in ManyChat but you don’t expect them to be triggered this way, please, open the live chat conversation with the user who experienced the issue and check if there are records about any triggers (like Rules, Sequences, etc) - this may clarify things!