why is 4 the outcome ?

"Roelof Wobben [email protected] [ocaml_beginners]" <[email protected]> Wed, 15 Oct 2014 14:17:40 +0200

Newsgroups	gmane.comp.lang.ocaml.beginners
Message-ID	<[email protected]>

Hello,

I have this programm :

let x = 1 in let x = 2 in x + x

I was expected to see a outcome of 3 but when I run it  I see a outcome 
of 4 ,

Can someone explain to me why ??

Roelof

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