auto replace on show post

Hello guys,
I would like to know if there is any way I could loop the post for a custom tag (in my case I will look for the second <h2> in every post) and replace it with some code. I want to add something like “if h2!=0 and h2 is found in the post two times, replace the second one with include(‘file.inc’);<h2>”.

Thanks in advance,
i appreciate every suggestions