Help to eliminate duplicated from data.frame but Special Problem

Dear Reseacher,
i need to resolve the following problem. I wish to delete duplicate row from
a data.frame but not all duplicate row:

ex:

my.df <- data.frame(Id=c(1,2,3,4,5,5,6,7,8,8,8,9),
value1=c(10,20,30,40,50,50,60,70,80,80,81,90),
value2=c(100,200,300,400,500,500,600,700,800,800,799,900))

my.df

   Id value1 value2
1   1     10    100
2   2     20    200
3   3     30    300
4   4     40    400
5   5     50    500
6   5     50    500
7   6     60    600
8   7     70    700
9   8     80    800
10  8     80    800
11  8     81    799
12  9     90    900