A secondary user can't have the same email if a primary Hospitable.com account exists with that email.
You need to either use a different email address or contact us so we can migrate the account to being a secondary user on the new account. Please note that doing this will remove the existing primary account and you/they will lose all information associated with it.
You can however invite someone who is a secondary user on another primary account to be a secondary user on your account using the same email address.
When they login using this email address, they can switch their view between the different accounts they're secondary users for. The interface will show them only the data relevant to the selected account.