An implementation of ToDictionary that provides information about which key had duplicate entries.

ToDictionaryBetter.cs

public static class EnumerableExtensions
{
    public static Dictionary<TKey, TSource> ToDictionaryBetter<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
    {
        return source.GroupBy(keySelector).ToDictionary(group => group.Key, group =>
        {
            try
            {
                return group.Single();
            }
            catch (InvalidOperationException)
            {
                throw new InvalidOperationException(
                    string.Format("Duplicate item with the key '{0}'", keySelector(@group.First())));
            }
        });
    }
}