See my answer inline

1. Is it possible that two agents have same name in the system? In this case the field name cannot be used to uniquely identify agents.

A. Yes, extensions can have same name, and user can change their name also. So you should not rely on a user name as a unique user identifier.

2. Can extensionId be used to uniquely identify agents? If yes, then where can I get the extensionId for a particular agent, as this information available in notification only and not in Admin Console (at least I wasn't able to find it there). I assume that what is displayed in Ext. column is not actual extension as for user with the name Nikolai Chashchin Ext.101 is displayed while with the notification I get 315395004

A. Extension id is a unique identifier. It's an internal id and no one can change it. Remember that the extension id is different than the extension number. If you want to detect the extension id from the admin console, select a user and pay attention to the URL. E.g. https://service.devtest.ringcentral.com/application/users/users/default/186478004/settings/default

3. Are there any other options to get agent identify from the notification?

A. The best way to identify is the agent's phone number or the extension id. Using the extension id would be the best and you should add the extension id into your system database so you can do a look up for agent other info. To detect extension (agent) info from your RingCentral account, you can use the extension id and call this endpoint.

4. Is Admin Console the only way to enter users to the system. Is there a way to import users from Microsoft Active Directory f.ex?

A.You can use the admin console to manage users. Or you can use APIs to read and manage certain extension data.

Thank you, Phong Vu