What are orphaned blocks and are they common in fastcoin system
Orphaned blocks are still valid blocks but just end up not getting included in the main chain. They can occur when two miners (or pools) produce blocks at similar times. Generally, which ever has the best share, or hashrate, will get their's accepted by the network It is also partially due to the fast blocktime, I've seen many miners have misconfigured settings, such as queue time, scantime and expiry. When mining on a P2Pool node, the solved block is able to be propagated across that entire network which greatly reduces the chances of orphans.
You can google for "Orphaned blocks" and find lots of other input and other ways describing it for better understanding.
You can also read up on how P2Pool works here:
https://en.bitcoin.it/wiki/P2Pool