我有兴趣为那些仅在数据集中出现一次或两次的ID（Id = 3370和3360）创建新的观察值。因此，给定n = 1或n = 2，我需要一个函数来创建新行。
以下是数据：
df <- data.frame(Id = c(6431, 6431, 6431, 6431, 3066, 3066, 3066, 3371, 3371, 3371, 3370, 3360, 3360), Order = c(1, 2, 3, 4, 3, 2, 1, 2, 1, 3, 1, 1, 2), Service = c("C", "E", "F", "C", "C", "E", "F", "C", "E", "F", "C", "E", "C"), n=c(4,4,4,4,3,3,3,3,3,3,1,2,2))
> df
Id Order Service n
1 6431 1 C 4
2 6431 2 E 4
3 6431 3 F 4
4 6431 4 C 4
5 3066 3 C 3
6 3066 2 E 3
7 3066 1 F 3
8 3371 2 C 3
9 3371 1 E 3
10 3371 3 F 3
11 3370 1 C 1
12 3360 1 E 2
13 3360 2 C 2
下面是我希望数据看起来的样子（对于显示一次或两次的每个ID，S *已附加到数据帧中）。
dfS=data.frame(Id=c(6431,6431,6431,6431,3066,3066, 3066,3371,3371,3371,3370,3370,3360,3360,3360), Order=c(1,2,3,4,3,2,1,2,1,3,1,2,1,2,3), Service=c("C","E","F","C", "C","E","F","C", "E","F","C","S*","E","C", "S*"))
> dfS
Id Order Service
1 6431 1 C
2 6431 2 E
3 6431 3 F
4 6431 4 C
5 3066 3 C
6 3066 2 E
7 3066 1 F
8 3371 2 C
9 3371 1 E
10 3371 3 F
11 3370 1 C
12 3370 2 S*
13 3360 1 E
14 3360 2 C
15 3360 3 S*
在此使用什么适当的功能？