If a user is "Marketing Cloud Enabled" then it is not possible to add with the same Email-Adress another property from another company.
Any Idea to accept such an invite?
You can use an alternate email or an alias i.e. firstname.lastname@example.org if your email recognizes aliases. There is more information on the FAQ page under 'I was invited to a DTM company, who do I get an error that my password is wrong?' https://marketing.adobe.com/resources/help/en_US/dtm/c_faq-users-permissions.html
Let me know if that doesn't work.