1. Why would an individual person see more than one of your ads. That’d be a pretty fundamental failure of Facebook
2. There’s a thousand ways they could be doing it. And they don’t disclose any of it.
The pixel does not track by itself. Think of it like a Trojan horse. It opens up a session that handles the tracking payload.
When the pixel is called, a browser session is established. Even though the page is served by a different server, the pixel served by FB allows FB to log the session. FB or any adserver that serves pixels already knows which ad was served to that user. It picks up session and device info such as page where the pixel is called, the website details, other pages viewed in session if the pixel was served to the same user elsewhere, URLs served, ads seen, device type, retargeting profile (if third party cookie is enabled on the user’s browser), etc. This info is mapped to the respective business account for attribution. One pixel should be enough to track multiple ads for a campaign, or even for multiple campaigns for a business.