sum() condition in ifelse statement

This question is related to this question My question is about R: How to number each repetition in a table in R?

Where basically the repetitions are numbered. E.g two repetitions: 1,2 ; three repetitions: 1,2,3 etc... But if the value is unique (only one time) it should have not 1 but NA

data: (from akrun, many thanks!)

df1 <- structure(list(Fullname = c("Peter", "Peter", "Alison", "Warren", 
                                   "Jack", "Jack", "Jack", "Jack", "Susan", "Susan", "Henry", "Walison", 
                                   "Tinder", "Peter", "Henry", "Tinder")), row.names = c(NA, -16L
                                   ), class = "data.frame")

my solution would be this:

df1 %>% 
  group_by(Fullname) %>% 
  mutate(newcol = seq_along(Fullname)) 

  Fullname newcol
   <chr>     <int>
 1 Peter         1
 2 Peter         2
 3 Alison        1
 4 Warren        1
 5 Jack          1
 6 Jack          2
 7 Jack          3
 8 Jack          4
 9 Susan         1
10 Susan         2
11 Henry         1
12 Walison       1
13 Tinder        1
14 Peter         3
15 Henry         2
16 Tinder        2

Now I try to set each value that occurs once (e.g. Alison, Warren and Henry) to NAlike akrun did here My question is about R: How to number each repetition in a table in R?

My code is with a ifelse statement checking if the sum of the group is >1.

df1 %>% 
  group_by(Fullname) %>% 
  mutate(newcol = seq_along(Fullname)) %>% 
  mutate(newcol = ifelse(sum(newcol)>1, newcol, NA))

but I get:

 Fullname newcol
   <chr>     <int>
 1 Peter         1
 2 Peter         1
 3 Alison       NA
 4 Warren       NA
 5 Jack          1
 6 Jack          1
 7 Jack          1
 8 Jack          1
 9 Susan         1
10 Susan         1
11 Henry         1
12 Walison      NA
13 Tinder        1
14 Peter         1
15 Henry         1
16 Tinder        1

And I can't grasp why?