Code executed twice

Hello,

I am using php_insert for long time but recently (probably after update to 3.8.2 but there is no way I can check it) my code started to be executed twice.

Normally I use require_once for external script – and it works. But now script does not produce output – even file with echo "OK";
When I changed require_once into require PHP complained about redeclaration of function. Quick way around is using if (!function_exist('foo')){ function foo(){ ... } }

My script downloads content from other website it’s quite slow and executing it twice is really bad.

I beleive that there is something in wordpress core that checks content of article with executing [php_insert].
I know PHP well but I do not have idea how to debug which function is guilty except reading source code of everything in /wp-include

Is there anything I can do not to execute code twice by modyfying only plugin?

https://wordpress.org/plugins/insert-php/