Set like functionality

[Mark]

We would like to maintain a history of all product views by a given 
user. We are currently using a row key like USER_ID_ID/TIMESTAMP. This 
works however we would like to maintain a unique list of these users to 
product views.

So if i have rows like:
mark/1328731167014262  = { data => 'Product 123' }
mark/1328731162502304  = { data => 'Product 456' }
mark/1328731157711375  = { data => 'Product 789' }

And I view Product 789 again I want it to be like:

mark/1328731292355173  = { data => 'Product 789' }
mark/1328731167014262  = { data => 'Product 123' }
mark/1328731162502304  = { data => 'Product 456' }

So it basically replaces the old value. How can this be accomplished?

Thanks