Github user uncleGen commented on a diff in the pull request:

    https://github.com/apache/spark/pull/16827#discussion_r99738940
  
    --- Diff: core/src/main/scala/org/apache/spark/SparkConf.scala ---
    @@ -779,6 +781,30 @@ private[spark] object SparkConf extends Logging {
       }
     
       /**
    +   * check if the given config key-value is valid.
    +   */
    +  private def checkValidSetting(key: String, value: String, conf: 
SparkConf): Unit = {
    +    key match {
    +      case "spark.master" =>
    +        // First, there is no need to set 'spark.master' multi-times with 
different values.
    +        // Second, It is possible for users to set the different 
'spark.master' in code with
    +        // `spark-submit` command, and will confuse users.
    +        // So, we should do once check if the 'spark.master' already 
exists in settings and if
    +        // the previous value is the same with current value. Throw a 
IllegalArgumentException when
    +        // previous value is different with current value.
    +        val previousOne = try {
    +          Some(conf.get(key))
    +        } catch {
    +          case e: NoSuchElementException =>
    +            None
    +        }
    +        if (previousOne.isDefined && !previousOne.get.equals(value)) {
    +          throw new IllegalArgumentException("'spark.master' should not be 
")
    +        }
    --- End diff --
    
    here, we choose to throw IllegalArgumentException to fail job. Besides, we 
can also choose to log a warning in a gentle way.


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