Read in an arbitrary number of integers from the file <code>numbers.text>\</code> and print out the first duplicate found, or a statement that there were no duplicates. Use the 
STL <code>set<code> container (it's a template, just like <code>vector</code>). Unfortunately, the <code>contains</code> function for set is not available until C++ 20 
(and we don't support that version); in addition, the <code>find</code> function returns an iterator and we didn't get that far with those. However, there IS a <code>size</code>
function, and don't forget, if the element being inserted is already there, no further insertion is done (since a set cannot contain duplicates). Use the size function
to determine if the element was inserted, and from that whether there was a duplicate.