Add method to get unique items from a collection.
Add method to get unique items from a collection.

For cases where we want both the unique items as well as the duplicates,
this should be more performant than using LINQ.

mnaiman/duplicati,mnaiman/duplicati,mnaiman/duplicati,sitofabi/duplicati,mnaiman/duplicati,sitofabi/duplicati,duplicati/duplicati,mnaiman/duplicati,duplicati/duplicati,sitofabi/duplicati,duplicati/duplicati,sitofabi/duplicati,duplicati/duplicati,sitofabi/duplicati,duplicati/duplicati