How to implement IEqualityComparer to return distinct values?

An EqualityComparer is not the way to go – it can only filter your result set in memory eg:

var objects = yourResults.ToEnumerable().Distinct(yourEqualityComparer);

You can use the GroupBy method to group by IDs and the First method to let your database only retrieve a unique entry per ID eg:

var objects = yourResults.GroupBy(o => o.Id).Select(g => g.First());