首先按Analyte分组，然后您可以为每个组选择所有Units。

var condensed = originalList
    .GroupBy(item => item.Analyte)
    .Select(group => new AnalyteUnitPair()
    {
        Analyte = group.Key,
        Unit = string.Join(",", group.Select(item => $"{item.Unit}:{item.Unit}"))
    });

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这个怎么样

var listOfPairs = new List<AnalyteUnitPair>
{
    new AnalyteUnitPair { Analyte = "Ce",Unit = "g" },
    new AnalyteUnitPair { Analyte = "Ce",Unit = "%"},
    new AnalyteUnitPair { Analyte = "Ce",Unit = "ppm"},
    new AnalyteUnitPair { Analyte = "Dy",Unit = "kg"},
    new AnalyteUnitPair { Analyte = "Dy",Unit = "%"},
    new AnalyteUnitPair { Analyte = "Dy",Unit = "ppm"}
};

var analyteUnitsQuery =
    from pair in listOfPairs
    group pair by pair.Analyte into g
    select new AnalyteUnits
    {
        Analyte = g.Key,
        Units = string.Join(',', g.Select(pair => $"{pair.Unit}:{pair.Unit}"))
    };

foreach (var analyteUnits in analyteUnitsQuery)
{
    Console.WriteLine($"Analyte={analyteUnits.Analyte}, Units={analyteUnits.Units}");
}

public class AnalyteUnitPair
{
    public string Analyte { get; set; }
    public string Unit { get; set; }
}

public class AnalyteUnits
{
    public string Analyte { get; set; }
    public string Units { get; set; }
}

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