Welcome to the Airtable community!
Contacting Airtable’s customer support is the best option for resolving billing issues. While you wait for a response, here are some things to look into.
Every user should have a unique email address. If two people are using the same email address/account, they should appear as one person to Airtable, as Airtable uses email addresses to uniquely identify users.
I suggest you look at https://airtable.com/account to see the email addresses for the billable collaborators.Either the workspace or one of the bases may have been inadvertently shared with another user without your knowledge.
Also note that billing is per workspace. If you have multiple paid workspaces, it doesn’t matter if both workspaces have the same users. Each workspace is billed independently from other workspaces.