speed up data import to sql from windows forms - sql-server

Question

I've made a windows forms app that imports data to my sql database.
This is how data can look in the database
Problem If I now want to import more data and say that there is a entry in the new data that I'm importing that already exist in my sql data table, that is, an entry with the exact same value that already exist in my sql table. Then I would like to not import this particular entry and move on the next one.
The code I'm using for this is paintstakingly slow and I would like to find a quicker/better way of doing this. Please see #region DAX for the code and the try catch.
Here is the code that executes on my button click for import :)
Thanks!
private void dataImportButton_Click(object sender, EventArgs e)
{
Cursor.Current = Cursors.WaitCursor;
int candle = 0;
string candleStick = "";
foreach (var item in checkedListBox1.CheckedItems)
{
candleStick += item.ToString();
}
Regex regex = new Regex(#"\d*");
Match match = regex.Match(candleStick);
if (match.Success)
{
candle = Convert.ToInt32(match.Value);
}
string nameOfTable = string.Empty;
foreach (var item in checkedListBox2.CheckedItems)
{
nameOfTable += item.ToString();
}
string filePath = textBox1.Text;
var listaNerladdat = System.IO.File.ReadLines(filePath);
if (nameOfTable == "DAX")
#region DAX
{
foreach (var item in listaNerladdat)
{
var splitLista = item.Split(';').ToList();
if (splitLista.Count < 5)
continue;
var tmpSplitInfo = new DaxSuperTabell();
DateTime tmpDate;
if (DateTime.TryParse(splitLista[0], out tmpDate))
tmpSplitInfo.TimeStampMVCR = tmpDate;
double tmpX;
if (Double.TryParse(splitLista[1].Replace('.', ','), out tmpX))
tmpSplitInfo.HighPriceMVCR = tmpX;
if (Double.TryParse(splitLista[2].Replace('.', ','), out tmpX))
tmpSplitInfo.LowPriceMVCR = tmpX;
if (Double.TryParse(splitLista[3].Replace('.', ','), out tmpX))
tmpSplitInfo.OpenPriceMVCR = tmpX;
if (Double.TryParse(splitLista[4].Replace('.', ','), out tmpX))
tmpSplitInfo.ClosePriceMVCR = tmpX;
tmpSplitInfo.CandleStick = candle;
try{
_context.DaxSuperTabell.AddRange(tmpSplitInfo);
_context.SaveChanges();
}
catch{MessageBox.Show("This entry is a double")}
}
}
#endregion
}