title: Duplicate json property when converting java object to json string using jackson
body: <p>I have Pojo object, with getAsJson function to return Json string for this object.
  I use JsonProperty to define json properties in this object.
  Use writeValueAsString of ObjectMapper to write json string for this object.</p>
  
  <pre><code>import org.codehaus.jackson.JsonGenerationException;
  import org.codehaus.jackson.annotate.JsonIgnore;
  import org.codehaus.jackson.annotate.JsonIgnoreProperties;
  import org.codehaus.jackson.annotate.JsonProperty;
  import org.codehaus.jackson.map.JsonMappingException;
  import org.codehaus.jackson.map.ObjectMapper;
  
  @JsonIgnoreProperties(ignoreUnknown=true)
  public class LogLikeArticleDetail extends BaseObject {
      private static final long serialVersionUID = -2018373118257019033L;
      @JsonProperty("LikeArticleGUId")
      private String likeArticleGUId;
      @JsonProperty("UserId")
      private String userID;
      @JsonProperty("UserName")
      private String userName;
      @JsonProperty("IP") 
      private String ip;
      @JsonProperty("OS") 
      private String os;
      @JsonProperty("UserAgent") 
      private String userAgent;
      @JsonProperty("WebsiteCode") 
      private String websiteCode;
      @JsonProperty("ArticleId") 
      private String articleID;
      @JsonProperty("ATitle") 
      private String aTitle;
      @JsonProperty("CateAlias") 
      private String cateAlias;
      @JsonProperty("LikeStatus") 
      private String likeStatus;
      @JsonProperty("TimeStamp") 
      private Date timeStamp;
          //get, set....
          //....
          @JsonIgnore
      public String getAsJSON() throws JsonGenerationException, JsonMappingException,    IOException{
          ObjectMapper mapper = new ObjectMapper();
          return mapper.writeValueAsString(this) ; 
      }
  }
  </code></pre>
  
  <p>Now, i get result</p>
  
  <pre><code>public static void main(String[] args) throws JsonGenerationException, JsonMappingException, IOException {
          Calendar calendar =  Calendar.getInstance();
          LogLikeArticleDetail logLikeArticle = new LogLikeArticleDetail("1","2","3","4","5","6","7","8","what thing \"nothing\" show","10","11",calendar.getTime());
          System.out.println(logLikeArticle.getAsJSON());
      }
  </code></pre>
  
  <p>But the result's duplicated properties:</p>
  
  <pre><code>{"LikeArticleGUId":"1","UserId":"2","UserName":"3","IP":"4","OS":"5","UserAgent":"6","WebsiteCode":"7","ArticleId":"8","ATitle":"what thing \"nothing\" show","CateAlias":"10","LikeStatus":"11","TimeStamp":1352256727062,"_likeArticleGUId":"1","websiteCode":"7","likeStatus":"11","userID":"2","userName":"3","ip":"4","os":"5","userAgent":"6","articleID":"8","aTitle":"what thing \"nothing\" show","cateAlias":"10","timeStamp":1352256727062}
  </code></pre>
  
  <p>Show me what's occur in this problem ?</p>