Take a look at admin_user table. Pick one of the users which you want to duplicate, for example "admin" user (it probably have all permissions). Then do INSERT into admin_user table, insert all the values except user_id (which will be auto-generated) and pick username that you want (it cannot be the same as username which already exists).
Once you create such user (let's call it "john"), execute following query to set the password you want:
UPDATE admin_user SET password = CONCAT(SHA2('xxxxxxxxNewPassword', 256), ':xxxxxxxx:1') WHERE username = 'john';
For example, if you want to set password to Changeme1, execute following query:
UPDATE admin_user SET password = CONCAT(SHA2('xxxxxxxxChangeme1', 256), ':xxxxxxxx:1') WHERE username = 'john';