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The first uses generics notation which allows the compiler to check the usage of the object and do casts for you.

Read the API doc for the Set class. It's explained there. SelectItem needs to override equals() and hashcode() if you want to tell the Set class that two objects are "duplicates".

The Set class can't/doesn't look inside of an object.

Which are the duplicates? Your posts don't have any comments like:
<<<<<<<<< First of two
...
<<<<<<<<< Second that is duplicate of First

By definition a Set only contains unique elements. Where and how are you seeing duplicates?
Can you post what you are seeing that makes you think there are duplicates?

What is the definition of:...