Opt-In can be repeated for the number without any issue. If the subscriber has the Business API on their blocked sender list then they will need to remove it to receive messages. The API offers no ability to override the subscribers Block List.
Refer to the Opt-In-and-Outs section of the WhatsApp API Documentation to learn more.
Learn how you can send WhatsApp messages to your customers easily and securely using the Sinch WhatsApp Business API.