Find duplicated rows with original - r

Question

I can get duplicated rows in R on a data.table dt using
dt[duplicated(dt, by=someColumns)]
However, I would like to get pairs of duplicated rows and the "non-duplicates", for example consider dt:
col1, col2, col3
A B C1
A B C2
A B1 C1
Now, dt[duplicated(dt, by=c('col1', "col2")) would give me something along the lines of
col1, col2, col3
A B C2
I would like to get this together with the row that it did not chose to be duplicated, that is
col1, col2, col3
A B C1
A B C2
Speed comparison of answers:
> system.time(dt[duplicated(dt2, by = t) | duplicated(dt, by = t, fromLast = TRUE)])
user system elapsed
0.008 0.000 0.009
> system.time(dt[, .SD[.N > 1], by = t])
user system elapsed
77.555 0.100 77.703

Answer 1

I believe this is essentially a duplicate of this question, though i can see how you may not have found it...
...here's an answer building off the logic outlined in the referenced question:
dt <- read.table(text = "col1 col2 col3
A B C1
A B C2
A B1 C1", header = TRUE, stringsAsFactors = FALSE)
idx <- duplicated(dt[, 1:2]) | duplicated(dt[, 1:2], fromLast = TRUE)
dt[idx, ]
#---
col1 col2 col3
1 A B C1
2 A B C2
Since you are using data.table, this is probably what you want:
library(data.table)
dt <- data.table(dt)
dt[duplicated(dt, by = c("col1", "col2")) | duplicated(dt, by = c("col1", "col2"), fromLast = TRUE)]
#---
col1 col2 col3
1: A B C1
2: A B C2

Answer 2

You can easily achieve this just by using .N:
dt[, .SD[.N > 1], by = list(col1, col2)]
## col1 col2 col3
## 1: A B C1
## 2: A B C2
Edit:
You can also try to use binary search which is very efficient, though it seems like duplicated is still more efficient
setkey(dt[, indx := .N, by = list(col1, col2)], indx)[!J(1)]
## col1 col2 col3
## 1: A B C1
## 2: A B C2