How to ensure users can't sign up for multiple accounts?

1. How are multiple accounts detected?
2. Can more than one person be signed into a Gmail account?
3. Can 2 people access the same email account?

How are multiple accounts detected?

every device has a Mac address (physical address) which identifies the device over the internet. APP companies have the information of your device Mac address, so when you create multiple accounts they detect you.

Can more than one person be signed into a Gmail account?

To let multiple users sign in and allow the primary user to sign in to an unmanaged account, such as their personal Google Account, select Unrestricted user access (allow any user to be added to any other user's session).

Can 2 people access the same email account?

A shared email account is a single shared email address, often something like support@ or help@, that is used by multiple members of a team or company to send and receive email from a central inbox.