correct handling of duplicate rows in database in .NET

Use a sql statement that checks for the row before inserting it. Here is a simple example for a table called person with two columns, forename and surname which are checked for uniqueness:

/// <summary>
/// Insert a row into the person table
/// </summary>
/// <param name="connection">An open sql connection</param>
/// <param name="forename">The forename which will be inserted</param>
/// <param name="surname">The surname which will be inserted</param>
/// <returns>True if a new row was added, False otherwise</returns>
public static bool InsertPerson(SqlConnection connection, string forename, string surname)
{
    using (SqlCommand command = connection.CreateCommand())
    {
        command.CommandText =
            @"Insert into person (forename, surname)
                Select @forename, @surname
                Where not exists 
                    (
                        select 'X' 
                        from person 
                        where 
                            forename = @forename 
                            and surname=@surname
                    )";
        command.Parameters.AddWithValue("@forename", forename);
        command.Parameters.AddWithValue("@surname", surname);

        int rowsInserted = command.ExecuteNonQuery();

        // rowsInserted will be 0 if the row is already in the database
        return rowsInserted == 1;
    }
}

I think you should chose exception way. Just do something like that:

foreach(var elem in elemntsFromFile)
{
    try
    {
       context.sometable.Add(elem);
       context.SaveChanges();
    }
    catch
    {
    }
}

One moment. I dodnt like that db.saveChanges runs in every iteration, but it will on 100% will have better performance then "the way of select-first". It will work and work as well.

A simple way to ignore the duplicates is to create your unique index with option IGNORE_DUP_KEY=ON. You won't then incur the overhead of testing for duplicates or catching exceptions.

e.g.

CREATE UNIQUE NONCLUSTERED INDEX [IX_IgnoreDuplicates] ON [dbo].[Test]
(
    [Id] ASC,
    [Col1] ASC,
    [Col2] ASC
)
WITH (IGNORE_DUP_KEY = ON) 

Also you can then use BULK INSERT to efficiently load all of your data with automatic duplicate removal.

See CREATE INDEX