Filtering records that occur more than once with multiple variables

I have a sample dataset. My objective is to keep any records whose user_id and plan_id occur more than once. I understand that you can count the frequency of a variable in a column with

n_occur <- data.frame(table(test$user_id))

But how would one count the frequency of variables in two columns and then filter the original dataset by those that occur more than once? For example, here is my test dataset:

> test
   user_id plan_id hour
1        1      10    2
2        2      10    4
3        3      20   23
4        4      20   12
5        5      10    8
6        1      10   10
7        5      20    6
8        1      20    5
9        1      20   18
10       5      10    7
11       1      30    6

And here is the intended output:

> output
  user_id plan_id hour
1       1      10    2
2       5      10    8
3       1      10   10
4       1      20    5
5       1      20    8
6       5      10   17

and the data:

> dput(test)
structure(list(user_id = c(1, 2, 3, 4, 5, 1, 5, 1, 1, 5, 1), 
    plan_id = c(10, 10, 20, 20, 10, 10, 20, 20, 20, 10, 30), 
    hour = c(2, 4, 23, 12, 8, 10, 6, 5, 18, 7, 6)), .Names = c("user_id", 
"plan_id", "hour"), row.names = c(NA, 11L), class = "data.frame")

Any suggestions would be appreciated!