Occasionally a user asks for this, but we decided that sometimes people do want to list their dupes in their collection. Having the ability to list them as separate items rather than just a quantity is because not all dupes are equal in a collection. One item might be missing a manual, another might be sealed. Allowing separate listings allows for different descriptions to be added. The user can definitely choose to use a number in their notes field to indicate quantity, but we wanted to offer users the option.
Dupes also can happen in a sell list as well.