Customer E-Mails not getting checked for Duplication in Admin

[Watschman]

Description
When creating a new customer via the Admin Interface, it is not checked whether or not the specified E-Mail address is already in use whereas during registration, it is getting checked. This leads to various problems, e.g. the reset password functionality. Additionally it is a major problem for third party applications which are communicating via the Shopware API and rely on E-Mails being unique for customers.

Possible Solution
Use the same validation mechanism as the frontend registration.

Additional context
Tested on Shopware Version 6.3.2.1 Stable and 6.3.5.2 Stable

[Watschman]

Seems to be fixed with 6.4.1 (NEXT-13348) I haven't tested it personally but closing it anyway.