score:1

Could you please check your table with sp_help to see if there is a different collation in the columns.

There is also a very through explanation on that issue here

score:0

You could make the query a bit easier, and avoid the issue, and I think this should "work":

from a in dc.ContentList
where a.ContentID != null
select new { ID = a.ContentID, a.PriceClass, .. }).Union(
    from b in dc.ContentList
    where dc.ContentList.FirstOrDefault(a => a.ContentID == b.ContentID) == null)
    select new { ID = b.ContentID, b.PriceClass, .. });

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