When you receive duplicate messages after opting in to any of your pages, there's a high chance that two identical workflows got triggered. This often happens when you duplicate master workflows and publish the duplicates but miss out on setting the master workflows into draft. So in this case, both the master workflows and the duplicates are published. If these workflows contain a message action, then the message will be sent from both workflows when they get triggered, causing customers or leads to receive duplicate messages.
If you ask why it is needed to duplicate the workflows, you can check out this article: Why duplicate the funnels and workflows?
To get rid of the duplicate messages, simply go to your Automation section in GHL and set the Master Workflows to Draft.