Yet another reason why you shouldn't put Java code in a JSP. Based on the compiler message, it appears that the variable named "session" has already been declared. Either you did that, or (more likely) the "session" variable is one which is built into every JSP.
The straightforward solution would of course be to just call your "session" variable something else. But the proper solution would be to take that code out of the JSP entirely and put it in a servlet, or a helper class, or some other Java class.