[c#] How to find all duplicate from a List<string>?

If you are using LINQ, you can use the following query:

var duplicateItems = from x in list
                     group x by x into grouped
                     where grouped.Count() > 1
                     select grouped.Key;

or, if you prefer it without the syntactic sugar:

var duplicateItems = list.GroupBy(x => x).Where(x => x.Count() > 1).Select(x => x.Key);

This groups all elements that are the same, and then filters to only those groups with more than one element. Finally it selects just the key from those groups as you don't need the count.

If you're prefer not to use LINQ, you can use this extension method:

public void SomeMethod {
    var duplicateItems = list.GetDuplicates();
    …
}

public static IEnumerable<T> GetDuplicates<T>(this IEnumerable<T> source) {
    HashSet<T> itemsSeen = new HashSet<T>();
    HashSet<T> itemsYielded = new HashSet<T>();

    foreach (T item in source) {
        if (!itemsSeen.Add(item)) {
            if (itemsYielded.Add(item)) {
                yield return item;
            }
        }
    }
}

This keeps track of items it has seen and yielded. If it hasn't seen an item before, it adds it to the list of seen items, otherwise it ignores it. If it hasn't yielded an item before, it yields it, otherwise it ignores it.

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