Including interp_1d in a big C++ project

[blindProgrammer]

Hi, I have this issue when it comes to including the interp_1d header in a big c++ project. I have the following configuration, where each .hpp file shown below has a corresponding .cpp file:

myPulse.hpp:
#include "nr3.hpp"
#include "interp_1d.hpp"

calc1.hpp:
#include "myPulse.hpp"

calc2.hpp:
#include "myPulse.hpp

main.cpp:
#include "myPulse.hpp"
#include "calc1.hpp"
#include "calc2.hpp"
#include "nr3.hpp"
#include "interp_1d.hpp"

This returns the error: "66 duplicate symbols for architecture x86_64", where basically the contents of interp_1d.hpp are being read three times. The issue is resolved when I remove the interp_1d.hpp header from main and from myPulse.hpp header, and instead include it in myPulse.cpp, but this is a very dirty way of solving it and I still can only use it in myPulse.cpp.
I tried adding the following header guards to the interp_1d header:

#ifndef INTERP_1D_HPP
#define INTERP_1D_HPP

#endif

but it didn't fix the issue. Can someone help me with this issue? I basically want to use interp_1d techniques in multiple classes in my project but can't seem to be able to.