How to Remove Duplicate Strings from an Array in Kotlin

By the end of this page, you will understand how to remove duplicate values from a Kotlin array, especially an Array<String?>. You will learn the most common Kotlin approaches such as distinct(), converting to a Set, and when to choose each option. You will also see how null values behave and how order is affected.

In Kotlin, removing duplicates means keeping only unique values from a collection.

For an Array<String?>, duplicates can include:

• repeated strings like "apple"
• repeated null values

Kotlin provides collection functions that make this simple. The most beginner-friendly option is distinct().

val items = arrayOf("apple", "banana", "apple", null, null)
val unique = items.distinct()
println(unique) // [apple, banana, null]

Why this works

distinct() returns a new list containing only the first occurrence of each value. If the same value appears again later, Kotlin skips it.

This matters because duplicate removal is common in real programs:

• cleaning user input
• preparing API data
• filtering repeated tags or categories
• avoiding repeated processing

Important detail: arrays vs lists

In Kotlin, Array is not the same as List.

• Array<String?> is a fixed-size array
• distinct() returns a List<String?>

Think of duplicate removal like checking people into an event.

• You have a list of names arriving in order.
• A staff member keeps a record of who has already entered.
• If a name appears for the first time, it is allowed in.
• If the same name appears again, it is ignored.
• null is like a guest with no name tag: the first one is still recorded, but repeated unnamed entries are ignored.

So Kotlin's duplicate-removal process is basically:

1. read each item from left to right
2. remember what has already been seen
3. keep only the first appearance of each value

The most common ways to remove duplicates from an Array<String?> in Kotlin are below.

1. Use distinct()

val items = arrayOf("apple", "banana", "apple", null, "banana", null)
val unique = items.distinct()

println(unique) // [apple, banana, null]

• distinct() returns a List<String?>
• it keeps the first occurrence of each value
• it preserves order

2. Convert back to an array

If you need an array as the result:

val items = arrayOf("apple", "banana", "apple", null)
val uniqueArray = items.distinct().toTypedArray()

println(uniqueArray.contentToString()) // [apple, banana, null]

3. Use toSet()

val items = arrayOf(, , , )
 uniqueSet = items.toSet()

println(uniqueSet) 

Consider this example:

val items = arrayOf("cat", null, "dog", "cat", null, "bird")
val unique = items.distinct()
println(unique)

Let's trace it.

Initial array

["cat", null, "dog", "cat", null, "bird"]

Step 1: read "cat"

• not seen before
• keep it

Current result:

["cat"]

Step 2: read null

• not seen before
• keep it

Current result:

["cat", null]

Step 3: read "dog"

Removing duplicates from arrays and collections is common in many Kotlin applications.

User input cleanup

A form may collect repeated tags or categories.

val tags = arrayOf("kotlin", "android", "kotlin", null)
val uniqueTags = tags.distinct()

API response cleanup

Sometimes API data contains repeated values.

val cities = arrayOf("Paris", "Tokyo", "Paris")
val uniqueCities = cities.distinct()

Search history

You may want to show each search term only once.

val searches = arrayOf("phone", "laptop", "phone")
val uniqueSearches = searches.distinct()

Data import scripts

CSV or database imports often need duplicate filtering before saving.

val emails = arrayOf("a@example.com", "b@example.com", "a@example.com")
 uniqueEmails = emails.distinct()

In real Kotlin codebases, developers usually choose a duplicate-removal approach based on the result type they need.

Common pattern: distinct() for readable code

val uniqueNames = names.distinct()

This is the most readable and idiomatic choice when you want a list.

When an array is still required

val uniqueNamesArray = names.distinct().toTypedArray()

This is common when calling older APIs or code that specifically expects Array<String?>.

Validation before duplicate removal

Developers often clean data first, then deduplicate it.

val cleaned = names
    .filterNotNull()
    .map { it.trim() }
    .filter { it.isNotEmpty() }
    .distinct()

This pattern is common when processing form fields or imported text.

Early data normalization

In real projects, duplicate removal is often part of a pipeline:

val result = rawValues
    .filterNotNull()
    .map { it.lowercase() }
    .distinct()

This avoids treating "Kotlin" and "kotlin" as different if the business rule says they should match.

Here are common beginner mistakes when removing duplicates in Kotlin.

Mistake 1: Expecting distinct() to return an array

val items: Array<String?> = arrayOf("a", "a", null)
val unique: Array<String?> = items.distinct() // Error

Why it fails

distinct() returns a List<String?>, not an Array<String?>.

Fix

val unique: Array<String?> = items.distinct().toTypedArray()

Mistake 2: Forgetting that null is kept once

Some beginners expect all null values to disappear automatically.

val items = arrayOf("a", null, "a", null)
println(items.distinct()) // [a, null]

Fix

If you want to remove completely:

Here is how the main options compare.

// Original array
val items: Array<String?> = arrayOf("a", "b", "a", null, null)

// Remove duplicates -> List<String?>
val uniqueList = items.distinct()

// Remove duplicates -> Array<String?>
val uniqueArray = items.distinct().toTypedArray()

// Remove duplicates -> Set<String?>
val uniqueSet = items.toSet()

// Remove nulls first, then duplicates -> List<String>
val uniqueNonNull = items.filterNotNull().distinct()

// Normalize case, then remove duplicates
val uniqueLower = items.map { it?.lowercase() }.distinct()

Quick rules

• distinct() returns a new List
• it keeps the first occurrence of each value
• it preserves order
• repeated null values become a single null
• use toTypedArray() if you need an array again
• use filterNotNull() if null should be removed entirely

Common patterns

How do I remove duplicates from an array in Kotlin?

Use distinct().

val unique = items.distinct()

If you need an array result, use:

val uniqueArray = items.distinct().toTypedArray()

Does Kotlin distinct() work with null values?

Yes. It treats null as a normal value and keeps only one null if multiple are present.

Does distinct() preserve order in Kotlin?

Yes. It keeps values in the order of their first appearance.

What is the difference between distinct() and toSet() in Kotlin?

distinct() returns a List, while toSet() returns a Set. Both remove duplicates, but they produce different collection types.

How do I remove duplicates and also remove values?

• List in Kotlin — many array operations return lists, so this helps you understand result types.
• Set in Kotlin — sets store unique values and are closely related to duplicate removal.
• filterNotNull() — useful when you want to remove null values before deduplicating.
• map() — often used to normalize values before calling distinct().
• Array in Kotlin — helps explain why arrays and lists behave differently.
• Null safety in Kotlin — important for understanding String? and how nullable values are handled.
• Collection transformations in Kotlin — distinct(), map(), and filter() are part of this broader topic.