The platypus has a number of unique characteristics which set it apart from other animals. It has: Webbed feet with a retracting webbed membrane which can expose the claws, enabling the platypus to...
A platypus can kill another platypus, but this is very rare. Males will engage in territorial fighting, but they are not aggressive by nature.
No, there is no difference whatsoever. Platypus is a nick name for the duck-billed platypus.
No, the platypus is the only species. It is not actually known as the duck-billed platypus: this is just a nickname which only non-Australians use.
It isn't. The plural of platypus is platypuses, not platypi.