try this

lib/services/db/sync.js

@@ -259,45 +259,50 @@ ProcessSerial.prototype._transform = function(block, enc, callback) {
     return callback(err);
   }
 
-  async.whilst(
+  if(check) {
-    function() {
+    return self._process(block, callback);
-      return self.db.concurrentTip.__height < block.__height || self.db.tip.__height < block.__height - 1;
+  }
-    },
-    function(next) {
-      var nextCalled = false;
-      // wait until concurrent handler is ahead of us
-      self.db.once('addblock', function() {
-        if(!nextCalled) {
-          next();
-        }
-        nextCalled = true;
-      });
-      self.db.once('concurrentaddblock', function() {
-        if(!nextCalled) {
-          next();
-        }
-        nextCalled = true;
-      });
-    },
-    function() {
-      self.db.getSerialBlockOperations(block, true, function(err, operations) {
-        if(err) {
-          return callback(err);
-        }
 
-        operations.push(self.db.getTipOperation(block, true));
+  var processed = false;
 
-        var obj = {
+  self.db.on('addblock', function() {
-          tip: block,
+    if(!processed) {
-          operations: operations
+      processed = true;
-        };
+      return self._process(block, callback);
-
-        self.tip = block;
-
-        callback(null, obj);
-      });
     }
-  );
+  });
+
+  self.db.on('concurrentblock', function() {
+    if(!processed) {
+      processed = true;
+      return self._process(block, callback);
+    }
+  });
+
+  function check() {
+    return self.db.concurrentTip.__height < block.__height || self.db.tip.__height < block.__height - 1;
+  }
+};
+
+ProcessSerial.prototype._process = function(block, callback) {
+  var self = this;
+
+  self.db.getSerialBlockOperations(block, true, function(err, operations) {
+    if(err) {
+      return callback(err);
+    }
+
+    operations.push(self.db.getTipOperation(block, true));
+
+    var obj = {
+      tip: block,
+      operations: operations
+    };
+
+    self.tip = block;
+
+    callback(null, obj);
+  });
 };
 
 function ProcessConcurrent(highWaterMark, db) {