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If the serial number you entered is for eBahn version 2 and it says that it is already registered to another account, you will have the opportunity to prove that you are the owner of this product by providing the email address and password to which the product is currently registered.
Bentley Publishers
https://bentleypubs.atlassian.net/wiki/spaces/faqs/pages/436774905/Contact+Us
If the serial number you entered is for eBahn version 3, both of the serial numbers for that license will be registered to you.