HashSet allows duplicate item insertion – C#

I’m guessing that you are creating a new Quotes with the same values. In this case they are not equal. If they should be considered equal, override the Equals and GetHashCode methods.

public class Quotes{ 
    public string symbol; 
    public string extension

    public override bool Equals(object obj)
    {
        Quotes q = obj as Quotes;
        return q != null && q.symbol == this.symbol && q.extension == this.Extension;
    }

    public override int GetHashCode()
    {
        return this.symbol.GetHashCode() ^ this.extension.GetHashCode();
    }
}