duplicated()

duplicated(x, incomparables = FALSE, MARGIN = 1, fromLast = FALSE, ...)

duplicated() returns a logical vector indicating which elements (or rows) are duplicates. This function is fundamental for data cleaning and analysis.

Syntax

duplicated(x, incomparables = FALSE, MARGIN = 1, fromLast = FALSE, ...)

Parameters

Parameter	Type	Default	Description
x	vector/data.frame/array	—	Input object to process
incomparables	vector	FALSE	Values to treat as incomparable
MARGIN	integer	1	For arrays: 1 for rows, 2 for columns
fromLast	logical	FALSE	If TRUE, count from last occurrence
...	arguments	—	Additional arguments passed to methods

Examples

Basic usage with vectors

x <- c(1, 2, 2, 3, 3, 3, 4)
duplicated(x)
# [1] FALSE FALSE  TRUE FALSE  TRUE  TRUE FALSE

Working with character vectors

names <- c("Alice", "Bob", "Charlie", "Alice", "Diana", "Bob")
duplicated(names)
# [1] FALSE FALSE FALSE  TRUE FALSE  TRUE

# Get only unique names
names[!duplicated(names)]
# [1] "Alice"   "Bob"     "Charlie" "Diana"

With data frames

df <- data.frame(
  id = c(1, 2, 2, 3, 4, 1),
  name = c("A", "B", "B", "C", "D", "A")
)

duplicated(df)
# [1] FALSE FALSE  TRUE FALSE FALSE  TRUE

Using fromLast argument

# Check from last occurrence instead of first
x <- c(1, 2, 2, 3)
duplicated(x, fromLast = TRUE)
# [1]  TRUE FALSE FALSE FALSE

Common Patterns

Remove duplicates from data frame

# Base R approach - keep first occurrence
df_unique <- df[!duplicated(df), ]

# Using dplyr
library(dplyr)
df_unique <- distinct(df)

Find duplicate indices

# Find first duplicate index
which(duplicated(x))
# [1] 3 4

# Find all duplicates (including repeated)
anyDuplicated(x)
# [1] 3

Filter to keep non-duplicated rows

# Keep only rows that haven't been seen before
df[!duplicated(df), ]

See Also

• base::unique
• base::match