copy bytes without memcpy()

It's a home work. I want to implement memcpy(). I was told memory area must not overlap. Actually I don't understand what that means, because this code works fine, but there is a possibility of memory overlap. How to prevent it?

void *mem_copy(void *dest, const void *src, unsigned int n) {
    assert((src != NULL) && (n > 0));  
    int i = 0;
    char *newsrc = (char*)src;
    char *newdest = (char*)dest;
    while (i < n) {
        newdest[i] = newsrc[i];
        i++;
    }
    newdest[i]='\0';
    return newdest;
}