better path validation

merger/src/main/java/com/metamx/druid/merger/coordinator/RemoteTaskRunner.java

@@ -498,16 +498,15 @@ public class RemoteTaskRunner implements TaskRunner, TaskLogProvider
       throw new ISE("Length of raw bytes for task too large[%,d > %,d]", rawBytes.length, config.getMaxNumBytes());
     }
 
-    cf.create()
-      .withMode(CreateMode.EPHEMERAL)
-      .forPath(
-          JOINER.join(
-              config.getIndexerTaskPath(),
-              theWorker.getHost(),
-              task.getId()
-          ),
-          rawBytes
-      );
+    String taskPath = JOINER.join(config.getIndexerTaskPath(), theWorker.getHost(), task.getId());
+
+    if (cf.checkExists().forPath(taskPath) == null) {
+      cf.create()
+        .withMode(CreateMode.EPHEMERAL)
+        .forPath(
+            taskPath, rawBytes
+        );
+    }
 
     runningTasks.put(task.getId(), pendingTasks.remove(task.getId()));
     log.info("Task %s switched from pending to running", task.getId());