//這里假如說有四個文本框

string mainseat = this.textBox1.Text;
string nextseat = this.textBox2.Text;
string storeseat1 = this.textBox3.Text;
string storeseat2 = this.textBox4.Text;
if (mainseat != "")
{
     list.Add(mainseat);
}
if (nextseat != "")
{
      list.Add(nextseat);
}
if (storeseat1 != "")
{
      list.Add(storeseat1);
}
if (storeseat2 != "")
{
       list.Add(storeseat2);
}    
if (list.Distinct().Count<string>() != list.Count)//排查數組中是否有重復元素
{
       //此時說明有重復
}          
else

       //說明沒有重復