Header Guard for Multiple Use of vex.h Header File?

Our teams use the vex.h header file for declaring global functions/variables and include vex.h in all of our source files. Recently I noticed that there’s no header guard in vex.h, but I was wondering if we should add one since we include it multiple times. We do not get any compiler errors/warnings and have not noticed any issues. Is the compiler automatically ignoring subsequent declarations? Are there potential issues if we don’t add a guard?

Thanks for any clarification and recommendations you can offer.