How to keep only first value from distinct values in one column based on repeated values in other column in R?

Question

So I have a large dataset that looks something like this:

df <- data.frame(ID =c("A","A","A","B","B","B","C","C","C"),
plot = c(1,1,1,2,2,3,4,5,5))

df

  ID plot
1  A    1
2  A    1
3  A    1
4  B    2
5  B    2
6  B    3
7  C    4
8  C    5
9  C    5

I want to keep just the first values from plot so the data looks like this in the end:

df1 <- data.frame(ID =c("A","A","A","B","B","B","C","C","C"),
plot = c(1,1,1,2,2,2,4,4,4))

df1

ID plot
1  A    1
2  A    1
3  A    1
4  B    2
5  B    2
6  B    2
7  C    4
8  C    4
9  C    4