In a thread (http://www.peercointalk.org/index.php?topic=2976.msg27609#msg27609) there is an attack described that this technically possible.
The basic idea is that the attacker use private addresses that once held lots of coins and coin age. Actually so much that the attacker using a customized version of a Peernode, could build a new blockchain history and broadcast this attack chain to the network.
The duplicate block check mechanism would not protect against this attack, since the private key that held the coins was not used for minting. The assumption is also that the attacker can gather enough private keys to make the attack possible, spending only very little resources on doing so.
Although I can only come up with pretty unlikely scenarios, the attack vector is interesting enough for other people to comment on (so I created this thread). learnmore writes: