Fix how duplicating an item works and looks

The way duplicating an item currently works is confusing. There is no indication that the duplication actually happened and you have to manually hunt for the duplicate.

How duplication should work:

• You select an item and hit "duplicate".
• The duplicate is added right after the item you duplicate. The duplicate is selected and visible. The duplicated item is no longer selected.

How it works:

• You select an item, and hit "duplicate".
• The item is duplicated, but no message or state change in the UI indicates that.

The duplicate is placed as sibling of the duplicated item, at the end of the list of siblings. It is not selected and might even not be visible.