//initialize a dictionary with keys and values.   
Dictionary<int, string> plants = new Dictionary<int, string>() {   
    {1,"Speckled Alder"},   
    {2,"Apple of Sodom"},   
    {3,"Hairy Bittercress"},   
    {4,"Pennsylvania Blackberry"},   
    {5,"Apple of Sodom"},   
    {6,"Water Birch"},   
    {7,"Meadow Cabbage"},   
    {8,"Water Birch"}   
}; 

Response.Write("<b>dictionary elements........ www.49028c.com </b><br />");

//loop dictionary all elements  
foreach (KeyValuePair<int, string> pair in plants) 
{
    Response.Write(pair.Key + "....."+ pair.Value+"<br />");
} 

//find dictionary duplicate values. 
var duplicateValues = plants.GroupBy(x => x.Value).Where(x => x.Count() > 1);

Response.Write("<br /><b>dictionary duplicate values..........</b><br />");

//loop dictionary duplicate values only           
foreach(var item in duplicateValues) 
{
    Response.Write(item.Key+"<br />");
}