remove duplicate line quickly

Hi, I have a huge text file which has millions lines. I want to delete the duplicate line if existing.
The line format is like
2 rs12345 4567 A G T C C T

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space seperator
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zhshqzycAuthor Commented:

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is duplicated here.
You can try something like this.  It assumes the 2nd field (rs12345, in your example) is the key.

var keys = new HashSet<string>();

var input = new StreamReader(@"C:\temp\inputfile.txt");
var output = new StreamWriter(@"C:\temp\outputfile.txt");

for (var line = input.ReadLine(); line != null; line = input.ReadLine())
	var key = line.Split(' ')[1];
	if (!keys.Contains(key))


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zhshqzycAuthor Commented:
Can I use linq?
Can you check my code?
var dataNoDups = (from line in data
                  let elements = line.Split(new char[] {' '}, StringSplitOptions.RemoveEmptyEntries)
                  group line by elements[1] into lineGroup
                  from ele in lineGroup
                   select ele).Distinct().ToArray(); 

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