how to remove unique entry and keep duplicates in R

Question

ID     Cat1  Cat2    Cat3   Cat4
A0001   358 11.25   37428   0
A0001   279 14.6875 38605   0
A0013   367 5.125   40152   1
A0014   337 16.3125 38624   0
A0020   367 8.875   37797   0
A0020   339 9.625   39324   0

I need help learning to how remove the unique rows in my file while keeping the duplicates or triplicates. For example, output should look like below:

ID     Cat1  Cat2    Cat3   Cat4
A0001   358 11.25   37428   0
A0001   279 14.6875 38605   0
A0020   367 8.875   37797   0
A0020   339 9.625   39324   0

If you can give me advice how to approach this problem, much appreciated.

Thanks for everyone's suggestions. I wanted to calculate the difference in value in the different Categories (i.e. Cat2, Cat 3) between the repeated measures (by unique ID). Would appreciate any suggestions.

Answer 1

Another option in base R Using duplicated

dx[dx$ID %in% dx$ID[duplicated(dx$ID)],]

#      ID Cat1    Cat2  Cat3 Cat4
# 1 A0001  358 11.2500 37428    0
# 2 A0001  279 14.6875 38605    0
# 5 A0020  367  8.8750 37797    0
# 6 A0020  339  9.6250 39324    0

data.table using duplicated

using duplicated and fromLast version you get :

library(data.table)
setkey(setDT(dx),ID) # or with data.table 1.9.5+: setDT(dx,key="ID")
dx[duplicated(dx) |duplicated(dx,fromLast=T)]

#       ID Cat1    Cat2  Cat3 Cat4
# 1: A0001  358 11.2500 37428    0
# 2: A0001  279 14.6875 38605    0
# 3: A0020  367  8.8750 37797    0
# 4: A0020  339  9.6250 39324    0

This can be applied to base R also but I prefer data.table here for syntax sugar.