readline: Use one history item for reentered line

If the command entered is exactly the same as the last history item,
don't dupe it in the history

lib/readline.js

@@ -188,12 +188,14 @@ Interface.prototype._onLine = function(line) {
 Interface.prototype._addHistory = function() {
   if (this.line.length === 0) return '';
 
-  this.history.unshift(this.line);
-  this.historyIndex = -1;
+  if (this.history.length === 0 || this.history[0] !== this.line) {
+    this.history.unshift(this.line);
 
-  // Only store so many
-  if (this.history.length > kHistorySize) this.history.pop();
+    // Only store so many
+    if (this.history.length > kHistorySize) this.history.pop();
+  }
 
+  this.historyIndex = -1;
   return this.history[0];
 };