C++ Multiple definitions even though only one definition is given

I'm trying to make a small unordered map that I can use to store keys and values permanently, as a sort of dictionary. The type TengwarChar is just an object with two values, a string and an enum, and later I'll add methods too. The header I use is as follows:

#ifndef TENGWARLIBRARY_H
#define TENGWARLIBRARY_H

#include "tengwarchar.h"
#include <unordered_map>
#include <algorithm>
#include <string>

typedef std::unordered_map<std::string, TengwarChar> CharMap;

extern const CharMap numbers = {
    {"0", TengwarChar("ð", SHORT)},
    {"1", TengwarChar("ñ", SHORT)},
    {"2", TengwarChar("ò", SHORT)},
    {"3", TengwarChar("ó", SHORT)},
    {"4", TengwarChar("ô", SHORT)},
    {"5", TengwarChar("õ", SHORT)},
    {"6", TengwarChar("ö", SHORT)},
    {"7", TengwarChar("÷", SHORT)},
    {"8", TengwarChar("ø", SHORT)},
    {"9", TengwarChar("ù", SHORT)}
 };

std::string translateFromEnglishToTengwar(std::string str);
std::string translateFromTengwarToEnglish(std::string str);

#endif // TENGWARLIBRARY_H

Then, I have a simple testing cpp file that goes along with it:

#include "tengwarlibrary.h"

std::string translateFromEnglishToTengwar(std::string str)
{
    std::transform(str.begin(), str.end(), str.begin(), (int (*)(int))std::tolower);
    return str;
}

std::string translateFromTengwarToEnglish(std::string str)
{
    return "Hello world.";
}

The problem is, I keep getting 'multiple definition of numbers[abi:cxx11]' errors, when invoke translateFromTengwarToEnglish in my main function, even though I'm fairly certain I have only defined it once, in my header file, using header guards too. In case it might help, here is my humble main cpp file:

#include "mainwindow.h"
#include <QApplication>

#include <iostream>

#include "utils/tengwarlibrary.h"

int main(int argc, char *argv[])
{

    std::string s = "BlaH FElfeFEJI, IEORlfj";
    std::cout<<translateFromEnglishToTengwar(s)<<std::endl;

    QApplication a(argc, argv);
    MainWindow w;
    w.show();

    return a.exec();
}