diff --git a/processing/src/main/java/io/druid/segment/incremental/IncrementalIndex.java b/processing/src/main/java/io/druid/segment/incremental/IncrementalIndex.java
index 9866618df7e..057a4b95ff2 100644
--- a/processing/src/main/java/io/druid/segment/incremental/IncrementalIndex.java
+++ b/processing/src/main/java/io/druid/segment/incremental/IncrementalIndex.java
@@ -152,14 +152,14 @@ public class IncrementalIndex implements Iterable<Row>
     }
 
     final List<String> rowDimensions = row.getDimensions();
-    String[][] dims = new String[dimensionOrder.size()][];
 
+    String[][] dims;
     List<String[]> overflow = null;
-    for (String dimension : rowDimensions) {
-      dimension = dimension.toLowerCase();
-      List<String> dimensionValues = row.getDimension(dimension);
-
-      synchronized (dimensionOrder) {
+    synchronized (dimensionOrder) {
+      dims = new String[dimensionOrder.size()][];
+      for (String dimension : rowDimensions) {
+        dimension = dimension.toLowerCase();
+        List<String> dimensionValues = row.getDimension(dimension);
         Integer index = dimensionOrder.get(dimension);
         if (index == null) {
           dimensionOrder.put(dimension, dimensionOrder.size());
@@ -175,6 +175,7 @@ public class IncrementalIndex implements Iterable<Row>
       }
     }
 
+
     if (overflow != null) {
       // Merge overflow and non-overflow
       String[][] newDims = new String[dims.length + overflow.size()][];
@@ -287,8 +288,9 @@ public class IncrementalIndex implements Iterable<Row>
       Aggregator[] prev = facts.putIfAbsent(key, aggs);
       if (prev != null) {
         aggs = prev;
+      } else {
+        numEntries.incrementAndGet();
       }
-      numEntries.incrementAndGet();
     }
 
     synchronized (this) {
diff --git a/processing/src/test/java/io/druid/segment/data/IncrementalIndexTest.java b/processing/src/test/java/io/druid/segment/data/IncrementalIndexTest.java
index 5a7b448a4a8..35fb2b81c0e 100644
--- a/processing/src/test/java/io/druid/segment/data/IncrementalIndexTest.java
+++ b/processing/src/test/java/io/druid/segment/data/IncrementalIndexTest.java
@@ -24,18 +24,59 @@ import io.druid.data.input.MapBasedInputRow;
 import io.druid.data.input.Row;
 import io.druid.granularity.QueryGranularity;
 import io.druid.query.aggregation.AggregatorFactory;
+import io.druid.query.aggregation.CountAggregatorFactory;
 import io.druid.segment.incremental.IncrementalIndex;
 import junit.framework.Assert;
 import org.junit.Test;
 
+import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Iterator;
+import java.util.List;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.TimeUnit;
 
 /**
  */
 public class IncrementalIndexTest
 {
 
+  public static IncrementalIndex createCaseInsensitiveIndex(long timestamp)
+  {
+    IncrementalIndex index = new IncrementalIndex(0L, QueryGranularity.NONE, new AggregatorFactory[]{});
+
+    index.add(
+        new MapBasedInputRow(
+            timestamp,
+            Arrays.asList("Dim1", "DiM2"),
+            ImmutableMap.<String, Object>of("dim1", "1", "dim2", "2", "DIM1", "3", "dIM2", "4")
+        )
+    );
+
+    index.add(
+        new MapBasedInputRow(
+            timestamp,
+            Arrays.asList("diM1", "dIM2"),
+            ImmutableMap.<String, Object>of("Dim1", "1", "DiM2", "2", "dim1", "3", "dim2", "4")
+        )
+    );
+    return index;
+  }
+
+  public static MapBasedInputRow getRow(long timestamp, int rowID, int dimensionCount)
+  {
+    List<String> dimensionList = new ArrayList<String>(dimensionCount);
+    ImmutableMap.Builder<String, Object> builder = ImmutableMap.builder();
+    for (int i = 0; i < dimensionCount; i++) {
+      String dimName = String.format("Dim_%d", i);
+      dimensionList.add(dimName);
+      builder.put(dimName, dimName + rowID);
+    }
+    return new MapBasedInputRow(timestamp, dimensionList, builder.build());
+  }
+
   @Test
   public void testCaseInsensitivity() throws Exception
   {
@@ -58,25 +99,52 @@ public class IncrementalIndexTest
     Assert.assertEquals(Arrays.asList("4"), row.getDimension("dim2"));
   }
 
-  public static IncrementalIndex createCaseInsensitiveIndex(long timestamp)
+  @Test
+  public void testConcurrentAdd() throws Exception
   {
-    IncrementalIndex index = new IncrementalIndex(0L, QueryGranularity.NONE, new AggregatorFactory[]{});
-
-    index.add(
-        new MapBasedInputRow(
-            timestamp,
-            Arrays.asList("Dim1", "DiM2"),
-            ImmutableMap.<String, Object>of("dim1", "1", "dim2", "2", "DIM1", "3", "dIM2", "4")
-        )
+    final IncrementalIndex index = new IncrementalIndex(
+        0L,
+        QueryGranularity.NONE,
+        new AggregatorFactory[]{new CountAggregatorFactory("count")}
     );
+    final int threadCount = 10;
+    final int elementsPerThread = 200;
+    final int dimensionCount = 5;
+    ExecutorService executor = Executors.newFixedThreadPool(threadCount);
+    final long timestamp = System.currentTimeMillis();
+    final CountDownLatch latch = new CountDownLatch(threadCount);
+    for (int j = 0; j < threadCount; j++) {
+      executor.submit(
+          new Runnable()
+          {
+            @Override
+            public void run()
+            {
+              try {
+                for (int i = 0; i < elementsPerThread; i++) {
+                  index.add(getRow(timestamp + i, i, dimensionCount));
+                }
+              }
+              catch (Exception e) {
+                e.printStackTrace();
+              }
+              latch.countDown();
+            }
+          }
+      );
+    }
+    Assert.assertTrue(latch.await(60, TimeUnit.SECONDS));
 
-    index.add(
-        new MapBasedInputRow(
-            timestamp,
-            Arrays.asList("diM1", "dIM2"),
-            ImmutableMap.<String, Object>of("Dim1", "1", "DiM2", "2", "dim1", "3", "dim2", "4")
-        )
-    );
-    return index;
+    Assert.assertEquals(dimensionCount, index.getDimensions().size());
+    Assert.assertEquals(elementsPerThread, index.size());
+    Iterator<Row> iterator = index.iterator();
+    int curr = 0;
+    while (iterator.hasNext()) {
+      Row row = iterator.next();
+      Assert.assertEquals(timestamp + curr, row.getTimestampFromEpoch());
+      Assert.assertEquals(Float.valueOf(threadCount), row.getFloatMetric("count"));
+      curr++;
+    }
+    Assert.assertEquals(elementsPerThread, curr);
   }
 }