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And one more thing:
You define the return value as char*, then you proceed to return "copy" with malloc, but if it fails, you return a const char*, which might be free'd. You should malloc this string.

char *_string_duplicate(const char *string)
{
    char *copy = (char*) malloc(sizeof(*copy) * (strlen(string) + 1));
    if (copy == NULL)
    {
        // return "Not enough memory for error message";
        const char* error_message = "Not enough memory for error message";
        size_t len = strlen(error_message);
        char* error = (char*) malloc(len*sizeof(char) + 1);
        strcpy(error, error_message);
        return error;
    }

    strcpy(copy, string);
    return copy;
}