R: Vlookup for a 'for' loop

i'm new to R and trying to use it in place of Excel (where i have more experience). I'm still working out the full 'for' logic, but not having the values to determine if it's working how i think it should is stopping me in my tracks.

I have a dataframe that looks like this

Person <- c("A", "B", "C", "C", "D", "E","E")
Date <- c(1/1/20, 1/1/20,12/25/19, 1/1/20, 1/1/20, 12/25/19, 1/1/20)
ID <- c(1,2,3,4,5,6,7)
DuplicateStatus <- c(0,0,0,0,0,0,0)
IdealResult <- c(0,0,1,2,0,1,2)
mydata <- cbind(Person, Date, ID, DuplicateStatus, IdealResult)

I am trying to use a for loop to evaluate if person duplicates. If a person does not duplicate, value= 0 and if they do duplicate, they should have a 1 for the oldest value and a 2 for the newest value (see ideal result). NOTE: I have already sorted the data to be by person and then date, so if duplicated, first appearance is oldest.

previous investigations of Vlookup in R answers here are aimed at merging datasets based on identical values in multiple datasets. Here, i am attempting to modify a column based on the relationship between columns, within a single dataset.

currentID = 0
  nextID =0

  for(i in mydata$ID){
    currentID = i 
    nextID = currentID++1

CurrentPerson ##Vlookup function that does - find currentID in ID, return associated value in Person column in same position.

NextPerson ##Vlookup function that does - find nextID in ID, return associated value in Person column in same position.

if CurrentPerson = NextPerson, then DuplicateStatus at ID associated with current person should be 1, and DuplicateStatus at ID associated with NextPerson = 2.

**This should end when current person = total number of people Thanks!