One-Definition Rule Followed, but C++ throws Redefinition Error

Very strange redefinition error in C++, especially as every other file including main is error-free.

I have my headers (various animals) and an implementation file "animals.cpp".

My headers follow the format:

class Mammal : public Animal{
    public:
        Mammal(){} //empty constructor
        virtual ~Mammal(){} // destructor
        void createNewMammalObject(std::string name, std::string trackingNum, std::string nurse, std::string subType, std::string type){}
        std::string getSubtype() {}
        void setSubtype(std::string subType){}
        int getNursing(){}
        void setNursing(int nursing){}
        void setType(std::string type){}
        int getNumEggs(){}

    protected:
        int nursing;
};

And implementation in the implementation file looks like:

Mammal::Mammal() {} //empty constructor

virtual Mammal::~Mammal(){} // destructor

void Mammal::createNewMammalObject(std::string name, std::string code,std::string nurse,std::string subType, std::string type){
    this->setNursing(nursing);
    this->setSubType(subType);
    this->createNewAnimalObject(name, trackingNum,subType,type);
}

std::string Mammal::getSubtype() {
    return subType;
}

void Mammal::setSubtype(std::string subType) {
    this->subType = subType;
}

int Mammal::getNursing() {
    return this->nursing;
}

void Mammal::setNursing(int nursing) {
    this->nursing = nursing;
}

void Mammal::setType(std::string type){
    this->type = type;
}

int Mammal::getNumEggs() {
    return 0;
}

My #includes for the implementation file are:

#include "animal.h"
#include "oviparous.h"
#include "mammal.h"
#include "crocodile.h"
#include "goose.h"
#include "pelican.h"
#include "bat.h"
#include "seaLion.h"
#include "whale.h"

All headers and implementation follow this format to follow the One-Definition, except for animal.h which is an abstract class and does contain function definitions. All other functions are definitely only defined once. However, after building the project, EVERY function in the implementation file is saying it's a redefinition and pointing back to the headers as the original definition. I'm incredibly confused. Is this an Eclipse problem? Should my abstract class be defined in my implementation file like the other headers?