public static IEnumerable <T> Duplicates <T>(this IEnumerable <T> collection, IEqualityComparer <T> comparer)
{
collection.AssertParameterNotNull(nameof(collection));
comparer.AssertParameterNotNull(nameof(comparer));
return(collection.GroupBy(o => o, comparer).Where(o => o.Count() > 1).Select(o => o.Key).ToList());
}
