If you get a large number of email open notifications (more than you may expect) for a specific tracked email, it may indicate that the email was forwarded or that you copied and pasted a previously sent email.
HubSpot tracks email opens by inserting an invisible image into every email you send. When that image is loaded, HubSpot knows that the email was opened and an open notification is triggered. If the email is forwarded, this will result in multiple opens. Due to privacy and security concerns around tracking who exactly an email is forwarded to, HubSpot Sales defaults to telling you that the original sender opened the email again.
Additionally, multiple notifications could happen if you copied and pasted an email that you previously had sent. This would inadvertently copy the old tracking pixel. When the email was opened, the same tracking pixel would load for each recipient. It is recommended to send personalized emails, which you can create from scratch in the CRM or your mail client, or to use sales email templates.
Sometimes, the tracking image will be loaded multiple times from a single email open due to how the recipient views their emails. In particular, a user could be on a server which may dynamically change that user's IP address. If the image is loaded from multiple IP addresses then a notifications may indicate that your emails were being opened by multiple people. Also, some email clients like (Outlook or Apple Mail) have a preview pane as part of the inbox view, and if recipients scroll over your email in their inbox (even passively), this can trigger an email open notification.