You can check whether there is any matched back or lay bets on the selection that you are going to bet on.
For example, if you're backing on the "Favourite" in HiLo, which means it will bet on the first available selection in each round, you can prevent it from placing two back bets by adding this condiition to the trigger:
Selection's Matched Back is equal to 0
If you're laying, use the same one, only with "Matched Lay".