You may need to enclose the definitions in the header by a set of directives like so
#if !defined(SOME_CONSTANTE_OF_YOUR_CHOICE) #define SOME_CONSTANTE_OF_YOUR_CHOICE // Your definitions here ... #endif
Event if those definitions are only found in one file, if the file is included in multiple other files (directly or indirectly), they will be difined multiple times.
I don't know Dev C++ nor the compiler you are using but #pragma once may also work.
I hope this helps.