This is how I did it:
dictionary.add(control, "string1");
dictionary.add(control, "string1");
dictionary.add(control, "string2");
int x = 0;
for (int i = 0; i < dictionary.Count; i++)
{
if (dictionary.ElementAt(i).Value == valu)
{
x++;
}
if (x > 1)
{
dictionary.Remove(control);
}
}