Weird error, you can try methods given by james goooseling and also I have done it by following ways:
Remove all jars or libraries from build path and lib folders and add them again in lib folder or build path as you wish.
Check if you have servlet annotation @WebServlet
for given servlet and if it has mapping in web.xml as well .. this is most common error due to which tomcat gets confused , that what url pattern must he use.