HubSpot deduplicates contacts based on usertoken. In other words, when someone visits your site, HubSpot drops a tracking cookie (usertoken) on their browser to identify them as a unique visitor. When one visitor (you or someone outside of your organization) fills out more than one form on behalf of multiple people, those submissions will be deduplicated as a result of that unique tracking cookie. Contacts that are entered manually in this way are likely merged under one contact record.
To see raw form submissions, navigate to Contacts > Forms. Click on the number in the Submissions column. This will show you all unique submissions to that form. Click on the contact's name to navigate to their contact record. If the contact record has a different name and information than the form submission itself, then the record has probably been deduplicated with other submissions.
The best way to manually add several contacts to HubSpot at once is with an import. This will prevent issues with deduplication.