We can, its obviously pretty trivial, I do explictly remove it. It does seem like its not really obviously in the contract though? 
the similar F# function would take the view it should go from T => T: http://msdn.microsoft.com/en-us/library/ee340464.aspx . datafu for pig appears to have the same operator: http://datafu.incubator.apache.org/docs/datafu/1.1.0/datafu/pig/bags/DistinctBy.html

I've no personal strong preference on this though