If you get a large number of email opens (more than you may expect) on a specific email, it may indicate that the email was forwarded or that you copied and pasted a previously sent email. These notifications may also be shown in vastly different locations in your activity feed.
HubSpot tracks email opens by inserting an invisible image into every email you send. When that image is loaded, we know the email was opened. The multiple open notifications is due to the forwarded email in this case. There are a lot of privacy and security issues around tracking who exactly an email is forwarded to, so 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 which would in turn cause the same behavior as an email forward. In the future, ensure you are sending new emails from scratch instead of copying and pasting or forwarding.
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.