I'm using this in a site but because there's no validation on the keys, I've been getting the dreaded error "An item with the same key has already been added."

Just add this to the KeyValueEditorValueConverter line 42

var uniqueValues = pairs.GroupBy(pair => pair.Key)
                     .Select(group => group.First())
                     .ToDictionary(pair => pair.Key, pair => pair.Value);

        foreach (var pair in uniqueValues)
        {
            retval.Add(pair.Key, pair.Value);
        }