Storage: File already written with different data

I run a example in STM32F4DISCOVERY board.
It's about Storage.
I copy codes into right-side IDE like this:

var s = require("Storage");
s.write("hwll","ddd");

then I send and save into board.
But it make error like this

Uncaught Error: File already written with different data
at line 1 col 21
s.write("hwll","ddd");

What's wrong with it?