I'am asking myself why should i use include or include\_once.

I know there is a question [about it](https://stackoverflow.com/questions/186338/why-is-require-once-so-bad-to-use), but i dont find my answer in it.

In my case my problem is a little bit different, but it can summarize as following.
If i use 4 files who includes themselves, by example:

A includes D

B includes C,D

C no include

D includes C

Should i use "include" or "include\_once", and why ?

**EDIT**

Little mistake in the problem. I just rewrite.

**EDIT**

To explain my self, D need C to works, but C does not need for D to works. Sometime i have to include C, sometimes i have to include D. D has been refactored, so now it includes C automatically, but there is more than 2 thousand files to review in many directories... so i'am wondering if i use a "include\_once" in D is better to use a "include" in D.