[R] how to create duplicated ID in multi-records per subject dataset

[Zhixin Liu]

Hi R helpers,

If I have a dataset looks like:
ID   record 
1        20
.         30
.         25
2         26
.         15
3         21
4.....................

And I want it becomes 
ID   record 
1        20
1        30
1        25
2         26
2        15
3         21
4.....................

That is, I have to duplicate IDs for those with multiple records. I am wondering it is possible to be done in R, and I am grateful if you would like to show me the direction.

Many thanks!

Zhixin