gistfile1.txt

var document = XDocument.Load(File.OpenRead(@"myfile.xml"));
var entities = document.Element("ImportExportXml").Element("Entities").Elements("Entity");
 
var result = 
    entities.Elements("Entity")
            .SelectMany(
                e => e.Element("EntityInfo").Element("attributes").Elements("attribute"))
            .Distinct();

result.Dump();