Try adding a hidden parameter to your script: set it to a pre-determined value when the script invokes itself and check in the beginning of the script if this parameter is set to that value.

Warning! This is not very secure as anyone able to determine the correct value of that parameter wil now have access to your script. As you can only find that value by coming from the trusted IP, the risk is probably rather low, but stil ...

CountZero

"If you have four groups working on a compiler, you'll get a 4-pass compiler." - Conway's Law