Identify duplicates and mark first occurrence and all others

Here's one approach of many:

m <- matrix(c(1,2,1,3,1,4,1,2,2,3,2,3,1,2,5), ncol = 3)

x <- table(m[,1])
as.character(m[,1]) %in% names(x)[x > 1]

## > as.character(m[,1]) %in% names(x)[x > 1]
## [1]  TRUE FALSE  TRUE FALSE  TRUE

# or wrap it up as function:

FUN <- function(vec) {
    x <- table(vec)
    as.character(vec) %in% names(x)[x > 1]
}

FUN(m[, 1])

## > FUN(m[, 1])
## [1]  TRUE FALSE  TRUE FALSE  TRUE

When I saw this question I asked myself "what would Jim Holtman or Bill Dunlap advise on Rhelp?". Haven't looked in the archives, but I think they might have advised using two "parallel" applications of duplicated, one with the defaults and one with the fromLast parameter and conjoining with a vector OR (|) operator.

duplicated(m[,1]) | duplicated(m[,1], fromLast=TRUE)
[1]  TRUE FALSE  TRUE FALSE  TRUE