The duplicated database probably appears as a drilldown to the main DB

If this is a hosted database:

a. Check your odoo.conf file for a parameter called "dbfilter". This parameter is used to filter the names of the databases visible in the list.

b. If your odoo.conf file does not contain this parameter, check the value of the parameter "--db-filter" that may be passed when Odoo is started from the command line.

If you are facing this issue for an Odoo online (SAAS) or Odoo.sh database, get in touch with Odoo support at https://www.odoo.com/help and they should be able to help you.

The answer posted by Muhammad Ahsan Maqbool is a good suggestion but does not solve the issue. I had previously tried to refresh the page, log out and back in, and view the database list from another user's account. But still the duplicate database would not appear in the list.

Further information: when I attempted to acces the "invisable" duplicate database via hard-typed URL, the database seemed to render in my webpage, but still didnt show up in the list. Also, the database had the alert box that states it needed confirmation via email to become active, but when I clicked the "didnt get the email?" link, a text box poped up that stated the database was already confirmed, and didnt let me input an email address. 

Note: in this example, the "hard-typed" URL would be https://database-b.odoo.com/web#cids=1&action=menu ​

Most likely once the database is duplicated you were supposed to refresh in order to reload the database from database server. If you are not reloading the page you will not see the newly created database

If this will help you please upvote if not please share more details and we will perform research to figure out the reason

Thanks