diff --git a/DIRECTORY.md b/DIRECTORY.md
index 3554092d..2c64065c 100644
--- a/DIRECTORY.md
+++ b/DIRECTORY.md
@@ -136,6 +136,7 @@
   * [Maxcharacter](./Strings/MaxCharacter.php)
   * [Reversestring](./Strings/ReverseString.php)
   * [Reversewords](./Strings/ReverseWords.php)
+  * [RemoveDuplicateCharacters](./Strings/RemoveDuplicateCharacters.php)
 
 ## Tests
   * Ciphers
diff --git a/Strings/RemoveDuplicateCharacters.php b/Strings/RemoveDuplicateCharacters.php
new file mode 100644
index 00000000..cdad3fb1
--- /dev/null
+++ b/Strings/RemoveDuplicateCharacters.php
@@ -0,0 +1,34 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * Removes duplicate characters from a string, retaining only the first occurrence of each character.
+ *
+ * @param string $inputString The input string from which duplicates will be removed.
+ * @return string The modified string with duplicate characters removed.
+ */
+
+function removeDuplicateCharacters(string $inputString): string
+{
+    // Initialize an empty array to keep track of seen characters
+    $seen = [];
+
+    // Initialize an empty string for the result
+    $result = '';
+
+    // Loop through each character in the input string
+    for ($i = 0; $i < strlen($inputString); $i++) {
+        $char = $inputString[$i];
+
+        // Check if the character has already been seen
+        if (!in_array($char, $seen, true)) {
+            // Add the character to the result and mark it as seen
+            $result .= $char;
+            $seen[] = $char;
+        }
+    }
+
+    return $result;
+}
+
diff --git a/composer.json b/composer.json
index 89db6716..d1512067 100644
--- a/composer.json
+++ b/composer.json
@@ -20,4 +20,3 @@
     "test": "vendor/bin/phpunit tests"
   }
 }
-
diff --git a/tests/Strings/RemoveDuplicateCharactersTest.php b/tests/Strings/RemoveDuplicateCharactersTest.php
new file mode 100644
index 00000000..6065ec81
--- /dev/null
+++ b/tests/Strings/RemoveDuplicateCharactersTest.php
@@ -0,0 +1,71 @@
+<?php
+
+declare(strict_types=1);
+
+require_once __DIR__ . '/../../vendor/autoload.php';
+require_once __DIR__ . '/../../Strings/RemoveDuplicateCharacters.php';
+
+use PHPUnit\Framework\TestCase;
+
+/**
+ * Unit tests for the removeDuplicateCharacters function.
+ * To run test: ./vendor/bin/phpunit tests/strings/removeDuplicateCharactersTest.php
+ */
+final class RemoveDuplicateCharactersTest extends TestCase
+{
+    /**
+     * Test with a string that has multiple duplicates.
+     */
+    public function testStringWithDuplicates(): void
+    {
+        $this->assertSame('progamin', removeDuplicateCharacters('programming'));
+    }
+
+    /**
+     * Test with a string that has no duplicates.
+     */
+    public function testStringWithoutDuplicates(): void
+    {
+        $this->assertSame('abc', removeDuplicateCharacters('abc'));
+    }
+
+    /**
+     * Test with an empty string.
+     */
+    public function testEmptyString(): void
+    {
+        $this->assertSame('', removeDuplicateCharacters(''));
+    }
+
+    /**
+     * Test with a string containing only one character repeated.
+     */
+    public function testSingleCharacterRepeated(): void
+    {
+        $this->assertSame('a', removeDuplicateCharacters('aaaaa'));
+    }
+
+    /**
+     * Test with special characters.
+     */
+    public function testStringWithSpecialCharacters(): void
+    {
+        $this->assertSame('ab!@', removeDuplicateCharacters('aabb!!@@'));
+    }
+
+    /**
+     * Test with a string containing spaces.
+     */
+    public function testStringWithSpaces(): void
+    {
+        $this->assertSame('a b', removeDuplicateCharacters('a a a b b'));
+    }
+
+    /**
+     * Test with a string containing mixed case characters.
+     */
+    public function testStringWithMixedCase(): void
+    {
+        $this->assertSame('aAbB', removeDuplicateCharacters('aAaAaAbBbB'));
+    }
+}
\ No newline at end of file