Buying additional carnet tickets

I have bought 10 carnet tickets for my smart card and have used most of these so i am reaching the point when i need to buy some more.

Do i need to wait until i have used the final ticket before i buy more (which would be a pain and risks me forgetting) or can i buy more before the last one is in use or has been used?

If the latter is possible then how does "the system" deal with the older tickets vs the newer ones, ie is there a danger if you top up before you have use your older ones that the oldest tickets will hit the 2 month lifetime limit and expire?

Hope this makes sense.....

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