[SPARK-18357] Fix yarn files/archive broken issue andd unit tests

## What changes were proposed in this pull request?

The #15627 broke functionality with yarn --files --archives does not accept any files.
This patch ensures that --files and --archives accept unique files.

## How was this patch tested?

A. I added unit tests.
B. Also, manually tested --files with --archives to throw exception if duplicate files are specified and continue if unique files are specified.

Author: Kishor Patil <kpatil@yahoo-inc.com>

Closes #15810 from kishorvpatil/SPARK18357.

yarn/src/main/scala/org/apache/spark/deploy/yarn/Client.scala

@@ -604,7 +604,7 @@ private[spark] class Client(
             cachedSecondaryJarLinks += localizedPath
           }
         } else {
-          if (localizedPath != null) {
+          if (localizedPath == null) {
             throw new IllegalArgumentException(s"Attempt to add ($file) multiple times" +
               " to the distributed cache.")
           }

yarn/src/test/scala/org/apache/spark/deploy/yarn/ClientSuite.scala

@@ -322,6 +322,23 @@ class ClientSuite extends SparkFunSuite with Matchers with BeforeAndAfterAll
     intercept[IllegalArgumentException] {
       clientArchives.prepareLocalResources(new Path(tempDirForArchives.getAbsolutePath()), Nil)
     }
+
+    // Case 4: FILES_TO_DISTRIBUTE can have unique file.
+    val sparkConfFilesUniq = new SparkConfWithEnv(Map("SPARK_HOME" -> libs.getAbsolutePath))
+      .set(FILES_TO_DISTRIBUTE, Seq(testJar.getPath))
+
+    val clientFilesUniq = createClient(sparkConfFilesUniq)
+    val tempDirForFilesUniq = Utils.createTempDir()
+    clientFilesUniq.prepareLocalResources(new Path(tempDirForFilesUniq.getAbsolutePath()), Nil)
+
+    // Case 5: ARCHIVES_TO_DISTRIBUTE can have unique file.
+    val sparkConfArchivesUniq = new SparkConfWithEnv(Map("SPARK_HOME" -> libs.getAbsolutePath))
+      .set(ARCHIVES_TO_DISTRIBUTE, Seq(testJar.getPath))
+
+    val clientArchivesUniq = createClient(sparkConfArchivesUniq)
+    val tempDirArchivesUniq = Utils.createTempDir()
+    clientArchivesUniq.prepareLocalResources(new Path(tempDirArchivesUniq.getAbsolutePath()), Nil)
+
   }
 
   test("distribute local spark jars") {