Using "include" twice

I can’t seem to get this right. It keeps messing up for me.

I have a PHP file that contains nothing but a “defined or die” and a class definition. I need to use this same class in two different places.

If I use “include” in both places, PHP doesn’t display an error, but it just gives me that “no formatting” look and doesn’t display everything.

Is there any way to check if a class has already been imported, or any way to allow you to import the same class twice without errors?