This is a behavior similar to the require() This is a behavior similar to the require() statement, with the only difference being that if the code from a file has already been included, it will not be included again.
The require_once() statement replaces itself with the specified file, much like the C preprocessor's #include works, and in that respect is similar to the require() statement. The main difference is that in an inclusion chain, the use of require_once() will assure that the code is added to your script only once, and avoid clashes with variable values or function names that can happen.