message: Do not call callback twice in case of error
  It doesn't seem like calling the callback twice is by design, so this change fixes that by putting the second call in an else block.
diff: @@ -772,13 +772,12 @@ export class DeltaManager extends EventEmitter implements IDeltaManager<ISequenc
  (err?: any) => {
  if (err) {
  callback(err);
  - }
  -
  - // We will queue a message to update our reference sequence number upon receiving a server operation.
  - // This allows the server to know our true reference sequence number and be able to correctly update
  - // the minimum sequence number (MSN). We don't acknowledge other message types similarly (like a min
  - // sequence number update) to avoid acknowledgement cycles (i.e. ack the MSN update, which updates the
  - // MSN, then ack the update, etc...).
  + } else {
  + // We will queue a message to update our reference sequence number upon receiving a server
  + // operation. This allows the server to know our true reference sequence number and be able to
  + // correctly update the minimum sequence number (MSN). We don't acknowledge other message types
  + // similarly (like a min sequence number update) to avoid acknowledgement cycles (i.e. ack the MSN
  + // update, which updates the MSN, then ack the update, etc...).
  if (message.type === MessageType.Operation ||
  message.type === MessageType.Propose) {
  this.updateSequenceNumber(message.type);
  @@ -788,6 +787,7 @@ export class DeltaManager extends EventEmitter implements IDeltaManager<ISequenc
  this.emit("processTime", endTime - startTime);
  callback();
  + }
  });
  }
  