It seems that what you described as "some business logic" is some common processing that you want to fire in both GET and POST methods. In that case you can refactor it as separate method.
If that logic is valid only inside that one controller you can make private method that is called by both Item() methods - private controller methods are not mapped to MVC actions.
If it is more application wide you can also consider refactoring it out to some helper class.
BTW - copy/paste should be banned in any development environment
"My software never has bugs. It just develops random features."