BIG-IP : iRule return statement

How can the same event exist more than once within a single iRule ?

It can't, but the same event can exist in multiple iRules applied to the same VIP.

Is 'current TCP connection' refer to a session that is maintained across multiple request-response sequences from a given client-browser ? Or does each new request initiate a new TCP connection ?

That generally depends on a few things, including the application and specific HTTP version. For example, prior to HTTP 1.1, unless a keep-alive was specifically established, a single HTTP request (and corresponding response) constituted a single TCP session. With 1.1 a TCP session can handle multiple HTTP requests. There's no real guarantee though, so always better to assume each HTTP request is atomic.