When sending an email, your AI Assistant will check if it has ever sent the exact same email to this lead before. If it has, it will flag an error and cancel the conversation. This is a safety feature to prevent accidental spamming of a lead.
Currently, there is no expiry on this requirement; once a lead has been sent an email your AI Assistant will never send them the exact same email again. If you need to reach out to the lead again, either update the messaging, or add some context-specific merge tags to your messaging to vary the email content.