var numbersList = project.Lists["spisokSchislami"];
var resultList = project.Lists["spisokSrezultatom"];
int con = 3;//sovpadeniya
Dictionary<string, int> bufer = new Dictionary<string, int>();
for(int i = 0; i < numbersList.Count; i++)
{
string num = numbersList[i];
if (bufer.ContainsKey(num))
{
int count = bufer[num];
count++;
bufer[num] = count;
if(count == con)
{
resultList.Add(num);
}
}
else
{
bufer.Add(num, 1);
}
}