[Serusers] How to handle retrasmissions?

What is the correct way to handle message retrasmissions?

Usually t_relay() seems to hadle them correctly. But if a transactions was previously created by some module function, I found that t_relay() (and subsequent sl_reply_error()) generates the following errors:

Dec 5 18:49:25 eowyn OpenSER[5654]: ERROR: t_newtran: transaction already in process 0x502a56f8 Dec 5 18:49:25 eowyn OpenSER[5654]: ERROR: sl_reply_error used: I'm terribly sorry, server error occurred (1/SL)

So, I have the following questions:

1) What is the correct script code to relay a message, even with retrasmissions and transaction already created?

2) Is it OK to simply discard retrasmissions (not forwarding or replying them)?

3) How can I recognize retrasmissions? Is t_lookup_request() the correct function for a test?

Thanks.