stream: remove dup property

PR-URL: https://github.com/nodejs/node/pull/13216
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

lib/_stream_writable.js

@@ -61,9 +61,6 @@ function WritableState(options, stream) {
   // if _final has been called
   this.finalCalled = false;
 
-  // if _final has been called
-  this.finalCalled = false;
-
   // drain event flag.
   this.needDrain = false;
   // at the start of calling end()