[R] sampling

[yf]

I want to sample from the ID. For each ID, i want to have 2 set of data. I
try the sample() function but it didn't work. 

> x<-data.frame(id=c(1,1,1,2,2,2,2,3,3,3,4,4), v1=c(1:12), V2=c(12:23))
> x
   id v1 V2
1   1  1 12
2   1  2 13
3   1  3 14
4   2  4 15
5   2  5 16
6   2  6 17
7   2  7 18
8   3  8 19
9   3  9 20
10  3 10 21
11  4 11 22
12  4 12 23
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