fatal error LNK1169: one or more multiply defined symbols found in game programming

The two int variables are defined in the header file. This means that every source file which includes the header will contain their definition (header inclusion is purely textual). The of course leads to multiple definition errors.

You have several options to fix this.

1. Make the variables static (static int WIDTH = 1024;). They will still exist in each source file, but their definitions will not be visible outside of the source file.

2. Turn their definitions into declarations by using extern (extern int WIDTH;) and put the definition into one source file: int WIDTH = 1024;.

3. Probably the best option: make the variables const (const int WIDTH = 1024;). This makes them static implicitly, and also allows them to be used as compile-time constants, allowing the compiler to use their value directly instead of issuing code to read it from the variable etc.

You can't put variable definitions in header files, as these will then be a part of all source file you include the header into.

The #pragma once is just to protect against multiple inclusions in the same source file, not against multiple inclusions in multiple source files.

You could declare the variables as extern in the header file, and then define them in a single source file. Or you could declare the variables as const in the header file and then the compiler and linker will manage it.