Accepted answer

If your schema is really this simply, it could be accomplished in a minimal amount of code using Tuple and HashSet<T>.

The basic strategy in any case is to create a data structure to track what you have seen and use that to determine what to output. A dictionary tracking counts could be used as well. However, as a means of memory versus code trade-off, I've chosen to use two sets instead of one dictionary:

// 1. Data structure to track items we've seen
var found = new HashSet<Tuple<string, int>>();

// 2. Data structure to track items we should output
var output = new HashSet<Tuple<string, int>>();

// 3. Loop over the input data, storing it into `found`
using (var input = File.OpenText(path))
    string line;
    while (null != (line = input.ReadLine()))
        // 4. Do your CSV parsing
        var parts = line.Split(','); // <- need better CSV parsing
        var item = Tuple.Create(parts[0], Int32.Parse(parts[1]));

        // 5. Track items we've found and those we should output
        // NB: HashSet.Add returns `false` if it already exists,
        // so we use that as our criteria to mark the item for output
        if (!found.Add(item)) output.Add(item);

// 6. Output the items
// NB: you could put this in the main loop and borrow the same strategy
// we used for `found` to determine when to output an item so that only
// one pass is needed to read and write the data.


Without knowing the exact specifics, the first step I would take is looking into a Linq To CVS library, such as this...

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