The function duplicated is a little unintuitive in that it returns TRUE for only one instance of the duplicated set. Take a close look at this.
library(tidyverse)
# this is how duplicated works
duplicated(c(1, 1, 2, 3, 4))
#> [1] FALSE TRUE FALSE FALSE FALSE
df <- data.frame(ID =c("DEV2962","KTN2252","KTN2252","ANA2548","DEV2698","HRT2921",NA,"KTN2624","ANA2548","ITI2535","DEV2732","HRT2837","ANA2548","KTN2542","ANA2813","ITI2210"),
city=c("del","mum","nav","pun","bang","chen","triv","vish","del","mum","bang","vish","bhop","kol","noi","gurg"),
Name= c("dev,akash","singh,rahul","abbas,salman","lal,ram","singh,nkunj","sharma,nikita","ali,sarman","singh,kunal","tomar,lakhan","thakur,praveen","ali,sarman","khan,zuber","singh,giriraj","sharma,lokesh","sharma,nikita","sharma,nikita"))
# you probably want this instead
df <- mutate(df, dup = Name %in% Name[duplicated(Name)])
df
#> ID city Name dup
#> 1 DEV2962 del dev,akash FALSE
#> 2 KTN2252 mum singh,rahul FALSE
#> 3 KTN2252 nav abbas,salman FALSE
#> 4 ANA2548 pun lal,ram FALSE
#> 5 DEV2698 bang singh,nkunj FALSE
#> 6 HRT2921 chen sharma,nikita TRUE
#> 7 <NA> triv ali,sarman TRUE
#> 8 KTN2624 vish singh,kunal FALSE
#> 9 ANA2548 del tomar,lakhan FALSE
#> 10 ITI2535 mum thakur,praveen FALSE
#> 11 DEV2732 bang ali,sarman TRUE
#> 12 HRT2837 vish khan,zuber FALSE
#> 13 ANA2548 bhop singh,giriraj FALSE
#> 14 KTN2542 kol sharma,lokesh FALSE
#> 15 ANA2813 noi sharma,nikita TRUE
#> 16 ITI2210 gurg sharma,nikita TRUE
Created on 2021-06-23 by the reprex package (v1.0.0)