Your problem is not a 'unique' identity problem, otherwise the list would tell you so with the error 'Key is not unique in collection'.

The thing about Listviews and keys is that even though you may convert '13' into a string, it still recognises it as a number (we'll call it cleverness shall we?).

If you want to use a numeric as the key, the best thing to do is append a single letter before the number you want to use...ie make the key 'K13' instead of just '13'.

Hope this helps you!