Logical inconsistence between Set and SortedSet interfaces

[Andrej Hollmann]

Hello,

I want to add few products to the SortedSet and sort them by price. I 
add four different elements to TreeSet:
["salt",0.5$], ["milk", 1$], ["bread", 1$], ["bananas", 2$]

But at the end my TreeSet contains only three elements:
["salt",0.5$], ["bread", 1$], ["bananas", 2$]

The "bread" replaced the "milk" because it has the same price. I think 
ordering and equity are different aspects and should be separated. A 
logically correct SortedSet implementation should contain all four 
elements in ordered style. In that way SortedSet would be downward 
compatible to inherited Set interface.

To test my thinks I wrote my own SortedSet, which is based on TreeList 
from apache-collection and achieved desired results.

PS: With new SortedSet I can sort products by date, or supplier, ... . I 
just need to implement my own comparator and can be sure that added 
products will be not replaced by others because elements are the same 
during comparison.

Best Regards
Andrej