Dear harisMDG,
Good day ,
Thanks for posting in Microsoft Community.
I understand that you have an concern on "creating shared mailbox". Before moving forward, I would like to draw your attention here the error "The proxy address "smtp:<shared mailbox name>" is already being used by the proxy addresses or LegacyExchangeDN of "<name>". Please choose another proxy address" you are getting this error because you are might be trying to putting the shared mailbox a name that's already in use.
This information can be verify with official KBs here : Error when creating shared mailboxes
Furthermore , If you would like to use same address then you need to fix conflict issue before creating the shared mailbox. Check for objects have the same proxy address, and then remove or change the proxy address of the object that's in conflict.
To do this and for complete steps check this KBs : Proxy address is already being used" error message in Exchange Online [ For this you may need to connect your PowerShell with global O365 admin credentials ]
If my suggested response helped you to provide sufficient information to narrow down your problem over this thread , do click on "Mark as Answer" and "Up-Vote" for the answer that helped you for benefit of the community and also for other community members.
NOTE : For your security and privacy , kindly don't mention any email address / password or other confidential information.
We look forward to your response. Thanks for your cooperation.
Sincerely,
Amul | Microsoft Community Moderator