Should I include a grad student who only duplicated my calculations as an author on an article?
There does not seem to be much arguments pro or against including him. 

But when in doubt, it's probably better to include the student them not.

I've been in that situation myself and I've decided to include the student. The arguments I used to convince myself to include him were as follows:


I could not really find any argument against including him; 
If I include him, he would be much more motivated in future works with me; whereas if I don't, he may get a little frustrated when he realizes that I published a paper exactly on the topic we were discussing without inviting him;
It looks good to whoever is looking at your interaction with students (graduate committee, department, etc) that you are publishing with them;
As the student should be learning to write his own papers, it's probably good that he starts doing this on a paper where he is not the main author.


So for me it was a win-win situation.

Of course, by include him I mean that you should invite him and have him help in writing the paper.