Specify behaviour when dealing with duplicate keys

[mattnathan]

I've been trying to find what the behaviour of different methods is with respect to duplicate keys, but can't find anything official or obvious.

Some limited testing has shown that duplicates are added to the list rather than replacing existing entries, but without it written into the specification I'm not sure whether I can rely on this or not. I've also noticed that Finding using a key that is duplicated seems to return the SkipListElement at the start of the duplicated block (so you can call Next to get the next, etc), again not sure I can rely on this behaviour or not.

For the record, I'd like duplicates to continue to be added.