The servlet API .jar file must not be embedded inside the webapp since, obviously, the container already has these classes in its classpath: it implements the interfaces contained in this jar.
The dependency should be in the provided
scope, rather than the default compile
scope, in your Maven pom:
<dependency>
<groupId>javax.servlet</groupId>
<artifactId>javax.servlet-api</artifactId>
<version>3.1.0</version>
<scope>provided</scope>
</dependency>