比较输入值到向量中的值

I'm trying to input a form of data validation in which when a user enter's a book's ISBN number, if it has already been stored then it will output an error message. However, I'm having trouble doing this. I'm not sure if I'm overloading the == operator correctly, and I'm not sure how to compare the vector values in the store_ISBN() function.I'm trying to input a form of data validation i