@betfair winner: In the conditions window, Just add the conditions
"Selection's" - "Number of Matched Lay Bets" - "is equal to" - "0".
and "Selection's" - "Number of Unmatched Lay Bets" - "is equal to" - "0"
You have to add this conditions to each of your lay triggers, so the trigger will check first whether a lay bet has been already placed on the selection. If yes, no new lay bet will be placed.
However, by using this method, there is a slight chance (depending on the trigger details) that the lay bet placed, might stay unmatched.
Another method is like this, add a trigger to your trigger block:
Action: Cancel Lay
Selections: All Selections
Conditions are met: "Selection's" - "Lay Matched" - "is greater than" - "0"