Get all related data but ignore if already fetched by records before

public static string[] GetRelatedData(string[] records)
{
    var relatedData = new List<string>();
    foreach (var record in records)
    {
        var data = GetRelatedData(record);
        foreach (var item in data)
        {
            if (!relatedData.Contains(item))
                relatedData.Add(item);
        }
    }

    return relatedData.ToArray();
}