while i'm bothering you with this silly questions, i've another doubt about inclusions

how to use #include? i mean it's different from java import, so using include(from what i remember) means include "piece of code"(correct if i'm wrong) and not just import a name space!

so, how to avoid duplicate include?
suppose i have main class, firstclass and secondclass, if i include secondclass in firstclass, and first class in main, does main class automatically include secondclass?

i hope u can understand what i'm trying to say