Multiple definition error (collect2: error: ld returned 1 exit status)

I was using https://github.com/jdduke/three_cpp as a header-only mode but faced some issues while compiling the same with my project.

The issue happens when I include the following matrix4.hpp header (only necessary part is included) in more than one C++ files

three/core/matrix4.hpp (removed unwanted parts)

#ifndef THREE_MATRIX4_HPP
#define THREE_MATRIX4_HPP

#include <three/common.hpp>

#include <three/core/math.hpp>
#include <three/core/quaternion.hpp>
#include <three/core/vector3.hpp>
#include <three/core/vector4.hpp>

namespace three {

class Matrix4 {
public:
  // Function without any erros
  THREE_DECL Matrix4();
  THREE_DECL Vector3    getScale() const;

  // Error causing functions
  Vector3 getColumnX() const;
  Vector3 getColumnY() const;
  Vector3 getColumnZ() const;
  Matrix4& setPosition( const Vector3& v );
};

} // namespace three

#if defined(THREE_HEADER_ONLY)
# include <three/core/impl/matrix4.ipp> // This is where all this functions is defined.
#endif // defined(THREE_HEADER_ONLY)

#endif // THREE_MATRIX4_HPP

three/core/impl/matrix4.ipp (removed unwanted parts)

#ifndef THREE_MATRIX4_IPP
#define THREE_MATRIX4_IPP

#include <three/core/matrix4.hpp>

namespace three {

Matrix4::Matrix4() {
  identity();
}

Matrix4::Matrix4( const Matrix4& other ) {
  copy( other );
}

Vector3 Matrix4::getScale() const {
  auto sx = Vector3( te[0], te[1], te[2] ).length();
  auto sy = Vector3( te[4], te[5], te[6] ).length();
  auto sz = Vector3( te[8], te[9], te[10] ).length();
  return Vector3( sx, sy, sz );
}

Vector3 Matrix4::getColumnX() const {
  return Vector3( te[0], te[1], te[2] );
}

Vector3 Matrix4::getColumnY() const {
  return Vector3( te[4], te[5], te[6] );
}

Vector3 Matrix4::getColumnZ() const {
  return Vector3( te[8], te[9], te[10] );
}

Vector3 Matrix4::getPosition() const {
  return Vector3( te[12], te[13], te[14] );
}

} // namespace three

#endif // THREE_MATRIX4_IPP

When three/core/matrix4.hpp is included from multiple C++ files following multiple definition error is generated

/usr/bin/ld: /tmp/ccmYWwMM.o: in function `std::_Rb_tree_const_iterator<std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const, bool> >::_Rb_tree_const_iterator(std::_Rb_tree_iterator<std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const, bool> > const&)':
/home/three/core/impl/matrix4.ipp:617: multiple definition of `three::Matrix4::setPosition(three::Vector3 const&)'; /tmp/ccSaHLVI.o:/home/three/three/core/impl/matrix4.ipp:617: first defined here
/usr/bin/ld: /tmp/ccmYWwMM.o: in function `three::Vector3::subSelf(three::Vector3 const&)':
/home/three/three/core/impl/matrix4.ipp:627: multiple definition of `three::Matrix4::getColumnX() const'; /tmp/ccSaHLVI.o:/home/three/three/core/impl/matrix4.ipp:627: first defined here
/usr/bin/ld: /tmp/ccmYWwMM.o: in function `three::Matrix4::getColumnY() const':
/home/three/three/core/impl/matrix4.ipp:631: multiple definition of `three::Matrix4::getColumnY() const'; /tmp/ccSaHLVI.o:/home/three/three/core/impl/matrix4.ipp:631: first defined here
/usr/bin/ld: /tmp/ccmYWwMM.o: in function `three::Matrix4::getColumnZ() const':
/home/three/three/core/impl/matrix4.ipp:635: multiple definition of `three::Matrix4::getColumnZ() const'; /tmp/ccSaHLVI.o:/home/three/three/core/impl/matrix4.ipp:635: first defined here
collect2: error: ld returned 1 exit status

So based on our tryouts we noted that THREE_DECL is not mentioned for the following functions in matrix4.hpp

• Vector3 getColumnX() const;
• Vector3 getColumnY() const;
• Vector3 getColumnZ() const;
• Matrix4& setPosition( const Vector3& v );

So after modifying these functions like follows errors are fixed

• THREE_DECL Vector3 getColumnX() const;
• THREE_DECL Vector3 getColumnY() const;
• THREE_DECL Vector3 getColumnZ() const;
• THREE_DECL Matrix4& setPosition( const Vector3& v );

Based on three/config.hpp in case of using a header-only version THREE_DECL is defined as inline.

What I didn't understand is

1. Why multiple definition error has occurred even if the same header is included in different compilation units?
2. What will be the significance of adding THREE_DECL for resolving these issues?

Any help will be appreciated.