How does the Fire only once per visitor trigger option work in Zendesk Chat?
The Fire only once per visitor feature only works when the Run trigger condition, When a visitor has loaded the chat widget, is selected.
The Fire only once per visitor option uses user cookies to prevent the trigger from firing more than once during the same session. A new user session that uses another web browser, an incognito browser, or a VPN, registers the visitor as a new user. This causes the trigger to fire again during the current chat session.
For more information on Chat triggers, see the article: Working with Chat triggers.