Currently 5%-10% of your bot subscribers may receive duplicated messages when opting-in through Ref URL only once. Our developers are actively working on investigating this matter together with Facebook.
The issue is coming from Facebook sending duplicate webhooks to ManyChat’s servers. Our team has filed a Facebook bug report and is actively helping Facebook with investigation of this issue. You can track the progress in this report: https://developers.facebook.com/support/bugs/239386000671228/
As a temporary workaround, we suggest building the Ref URL's opt-in message like this:
1. Starting step - Condition with 'Tag is 'Your Tag'' value
2. If the user meets this condition, they don't receive any message
3. If the user doesn't meet this condition, add 'Your Tag' and send them the actual opt-in message