Straight from the Javadocs for the ServletRequest API:
You should only use this method when you are sure the parameter has only one value. If the parameter might have more than one value, use getParameterValues(java.lang.String).
If you use this method with a multivalued parameter, the value returned is equal to the first value in the array returned by getParameterValues
Do you read the API documentation? You should.
http://java.sun.com/javaee/5/docs/api/