Currently it's not possible in the webadmin to see an overview of all created account aliases. The only way to find out which aliases are created is by scripting, or by opening every user account one by one and check the alises tab.
I ran into this when I created a new account and got the error that the account already existed, which was not the case. However, it turned out to be an alias assigned to an existing user account. The only way to find out which user account, was one of the above methods, which both are not very user-friendly.
Here's how this could work: in the webadmin, there could be an option 'Manage aliases' beneath 'Manage accounts'. This would show all created aliases, and the account they are linked to. This even could be a place to add new aliases. The would show up both here and in the aliases tab of the linked user account.