Indexing discussion (/ p1)

Hello everyone, I have a simple information on how to remove the search engines from indexing the two results the same but with different address?
In a nutshell:
Page topic: www.mysite.com/discussion/1/namediscussion
This same page is then indexed on google also in this way, creating a duplicate:
www.mysite.com/discussion/1/namediscussion/p1
How can I make this not happen?
thanks
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