C언어 multiple definition of (함수) 오류

#ifndef __FUNCTION_H__
#define __FUCNTION_H__
 
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
 
 
#define TRUE 1
#define FALSE 0
#define MAX_COMMAND_SIZE 100
 
char COMMAND[MAX_COMMAND_SIZE];
 
int what_is_command(char command[]);
 
void func_help(void);
 
 
 
#endif

#include "header.h"
 
int what_is_command(char command[]){
 
        if(strcmp(command,"help") == 0 || strcmp(command,"h") == 0)
                return 1;
 
        else if(strcmp(command, "dir")==0|| strcmp(command,"d") == 0)
                return 2;
 
        else if(strcmp(command,"quit" )==0|| strcmp(command,"q")==0)
                return 3;
        else if(strcmp(command,"history")==0|| strcmp(command,"hi")==0)
                return 4;
        else if(strcmp(command,"dump" )==0|| strcmp(command,"du")==0)
                return 5;
        else if(strcmp(command,"edit" )==0|| strcmp(command,"e")==0)
                return 6;
        else if(strcmp(command,"fill" )==0|| strcmp(command,"f")==0)
                return 7;
        else if(strcmp(command,"reset" )==0)
                return 8;
        else if(strcmp(command,"opcode" )==0)
                return 9;
        else if(strcmp(command,"opcodelist")==0)
                return 10;
        else
                return 0;
 
 
}
 
void func_help(void){
        printf("h[elp]\n");
        printf("d[ir]\n");
        printf("q[uit\n]");
        printf("hi[story\n");
        printf("du[mp] [start, end]\n");
        printf("e[dit] address, value\n");
        printf("f[ill] start, end, value");
        printf("reset");
        printf("opcode mnemonic\n");
        printf("opcodelist\n");
}

#include "header.h"
#include "define.c"
 
 
 
 
int main(){
 
    while(TRUE){
 
        printf("sicsim> ");
        scanf("%s", COMMAND);
 
        switch(what_is_command(COMMAND)){
                case 1 : func_help(); break;
                default: break;
     }
 
}
 
return 0;
}

main.out: main.o define.o
        gcc -o main.out main.o define.o
define.o: define.c header.h
        gcc -c define.c
main.o: main.c define.c header.h
        gcc -c main.c define.c