There is a restriction in Onport that requires emails to be unique to a vendor. This is to ensure there is only one login needed to access their dashboard. However, vendors may raise they want one email to access multiple portals under the same marketplace. For example, if one person is managing multiple brands under different Shopify stores.
As a workaround, you will need to either
Enter a unique email for each
Or, if the vendor is using gmail / gsuite, you can use the "+" sign in the email as a workaround. For example, [email protected] and [email protected] will both have email routed to [email protected]. This may not work with emails outside of Guite. So, you'll need to use option 1.
We do see the same vendor work with multiple marketplaces using Onport. If this is the case and they signed up with the same email on each, they can use the switcher in the top left of their portal to change between the marketplaces.