How to join same lines in textbox and count them

For every possible variation in line format, you have to create a parsing strategy; so, anything you can do to control/simplify/regularize the format is valuable. Consider:

private char[] lnsplitchars = new char[] { '.', ' '};

// note absence of error checking 'real-world code' should have !

private void btnParseLines_Click(object sender, EventArgs e)
{
    foreach (string line in textBox1.Lines)
    {
        string[] lnsplit = line.Split(splitchars, StringSplitOptions.RemoveEmptyEntries);

        int id = Int32.Parse(lnsplit[0]);

        string text = lnsplit[1];

        int ndups = 0;

        if (lnsplit.Length == 3)
        {
            ndups = Int32.Parse(lnsplit[2]);
        }
        
        // process line data
    }
}

The real work begins with you formulating a clear strategy on transforming the parsed lines: you attempt that, and show your code.

Perhaps you will need to create an intermediate data structure ... like an Array of Arrays ... to hold the parsed line data; and/or use Linq 'GroupBy to organize duplicates.