R how to detect first last name with different ID?

Question

ID <- c(1,2,2,3,3,3,4,5)
Firstname <- c("A","B","B","C","C","C","D","A")
Lastname <- c("F","G","G","H","H","H","I","F")
df <- data.frame(ID,Firstname,Lastname)

I want to find A F who has ID no 1 and 5. In my real data, it's a long list and there are some persons who use different ID no. with the same name. How would you find out the row index of these people? Any suggestion will be appreciated.

Answer 1

Getting the result as a list:

subset(aggregate(ID~Firstname + Lastname, unique(df), length), ID > 1)[1:2]

  Firstname Lastname
1         A        F