You must have packaged the servlet-api.jar along with the other libraries in your war file. You can verify this by opening up your war file and navigating to the WEB-INF/lib folder.
Ideally, you should not provide the servlet-api jar. The container, in your case Tomcat, is responsible for providing it at deploy time to your application. If you try to provide it as well, then issues arise due to version mismatch etc. Best practise is to just avoid packaging it. Remove it from the WEB-INF/lib.
Additional Information
If you are using maven for your packaging, then simply add the provided tag with the dependency and maven will make sure not to package it in the final war file. Something like
..
...
...
provided
