Multiple redirect/respond invocations not allowed ¿¿??

I think that the HTTP::redirect is executed twice because the rate limit iRule section is probably added to the configuration first and, the HTTP::redirect is not terminating the HTTP_REQUEST event.

1) I would use event priorities so that the waiting room section is executed first. This ensures that if the URI is "waitingroom", no rate limiting takes place and therefore no redirect. The lower numbered priority are execute first. https://devcentral.f5.com/wiki/iRules.priority.ashx

when HTTP_REQUEST priority 100 { waiting_room_section }  
when HTTP_REQUEST priority 200 { rate_limit_section }

2) Use a "return" or "event disable" immediately after the "HTTP::redirect".

BTW: check out this awesome iRule which is similar to what you are doing. https://devcentral.f5.com/articles/implementing-the-exponential-backoff-algorithm-to-thwart-dictionary-attacks.Uzv_0LRL-KI

HTH