A Form Submitted trigger without a form filter runs for every form submission in the account, so a registration for one event can fire another event's workflow and send that event's confirmation.
Open Automations > Workflows and find the workflow that sent the email.
Open the Form Submitted trigger and filter it to the specific event form. If the workflow should cover several forms, select all of them: multiple selections combine with OR behavior.
Confirm the workflow is Published, not Draft.
Payment workflows can fire the same way. Check any Payment Received trigger's filters, especially Payment Source, Sub-Source, Global Product, Payment status, and the product or price. Also check Order Submitted workflows: this trigger sits under Payment triggers and carries the purchased order data. Using Form Submission Triggers and Common automation mistakes cover trigger filters in detail.