Removing first duplicate dataframe
Here's a more efficient solution:

# an example data frame
results &lt;- data.frame(user = c(1,2,3,4,3,4,2,1,3,4,6,7,7), a = 1)

# the solution
results[duplicated(results$user), ]


How it works: duplicated returns a logical vector indicating whether a value was also present at a preceding position in the vector (for each value of results$user).

This logical index is used to choose the appropriate lines of the orginal data frame. This is achieved by using this vector as the first argument for [ and using an empty second argument (to select all columns).

The result:

   user a
5     3 1
6     4 1
7     2 1
8     1 1
9     3 1
10    4 1
13    7 1