diff --git a/src/Components/Blocks/Header.php b/src/Components/Blocks/Header.php
index cbdfae1..c3859fe 100644
--- a/src/Components/Blocks/Header.php
+++ b/src/Components/Blocks/Header.php
@@ -6,6 +6,7 @@ use Erusev\Parsedown\AST\Handler;
 use Erusev\Parsedown\AST\StateRenderable;
 use Erusev\Parsedown\Components\Block;
 use Erusev\Parsedown\Configurables\HeaderSlug;
+use Erusev\Parsedown\Configurables\SlugRegister;
 use Erusev\Parsedown\Configurables\StrictMode;
 use Erusev\Parsedown\Html\Renderables\Element;
 use Erusev\Parsedown\Parsedown;
@@ -98,9 +99,10 @@ final class Header implements Block
             /** @return Element */
             function (State $State) {
                 $HeaderSlug = $State->get(HeaderSlug::class);
+                $Register = $State->get(SlugRegister::class);
                 $attributes = (
                     $HeaderSlug->isEnabled()
-                    ? ['id' => $HeaderSlug->transform($this->text())]
+                    ? ['id' => $HeaderSlug->transform($Register, $this->text())]
                     : []
                 );
 
diff --git a/src/Components/Blocks/SetextHeader.php b/src/Components/Blocks/SetextHeader.php
index dbdf04b..482e37a 100644
--- a/src/Components/Blocks/SetextHeader.php
+++ b/src/Components/Blocks/SetextHeader.php
@@ -7,6 +7,7 @@ use Erusev\Parsedown\AST\StateRenderable;
 use Erusev\Parsedown\Components\AcquisitioningBlock;
 use Erusev\Parsedown\Components\Block;
 use Erusev\Parsedown\Configurables\HeaderSlug;
+use Erusev\Parsedown\Configurables\SlugRegister;
 use Erusev\Parsedown\Html\Renderables\Element;
 use Erusev\Parsedown\Parsedown;
 use Erusev\Parsedown\Parsing\Context;
@@ -90,9 +91,10 @@ final class SetextHeader implements AcquisitioningBlock
             /** @return Element */
             function (State $State) {
                 $HeaderSlug = $State->get(HeaderSlug::class);
+                $Register = $State->get(SlugRegister::class);
                 $attributes = (
                     $HeaderSlug->isEnabled()
-                    ? ['id' => $HeaderSlug->transform($this->text())]
+                    ? ['id' => $HeaderSlug->transform($Register, $this->text())]
                     : []
                 );
 
diff --git a/src/Configurables/HeaderSlug.php b/src/Configurables/HeaderSlug.php
index 2f32699..a0bc3c3 100644
--- a/src/Configurables/HeaderSlug.php
+++ b/src/Configurables/HeaderSlug.php
@@ -12,12 +12,19 @@ final class HeaderSlug implements Configurable
     /** @var \Closure(string):string */
     private $slugCallback;
 
+    /** @var \Closure(string,int):string */
+    private $duplicationCallback;
+
     /**
      * @param bool $enabled
      * @param (\Closure(string):string)|null $slugCallback
+     * @param (\Closure(string, int):string)|null $duplicationCallback
      */
-    public function __construct($enabled, $slugCallback = null)
-    {
+    public function __construct(
+        $enabled,
+        $slugCallback = null,
+        $duplicationCallback = null
+    ) {
         $this->enabled = $enabled;
 
         if (! isset($slugCallback)) {
@@ -32,6 +39,14 @@ final class HeaderSlug implements Configurable
         } else {
             $this->slugCallback = $slugCallback;
         }
+
+        if (! isset($duplicationCallback)) {
+            $this->duplicationCallback = function (string $slug, int $duplicateNumber): string {
+                return $slug . '-' . \strval($duplicateNumber-1);
+            };
+        } else {
+            $this->duplicationCallback = $duplicationCallback;
+        }
     }
 
     /** @return bool */
@@ -40,9 +55,23 @@ final class HeaderSlug implements Configurable
         return $this->enabled;
     }
 
-    public function transform(string $text): string
+    public function transform(SlugRegister $SlugRegister, string $text): string
     {
-        return ($this->slugCallback)($text);
+        $slug = ($this->slugCallback)($text);
+
+        if ($SlugRegister->slugCount($slug) > 0) {
+            $newSlug = ($this->duplicationCallback)($slug, $SlugRegister->mutatingIncrement($slug));
+
+            while ($SlugRegister->slugCount($newSlug) > 0) {
+                $newSlug = ($this->duplicationCallback)($slug, $SlugRegister->mutatingIncrement($slug));
+            }
+
+            return $newSlug;
+        }
+
+        $SlugRegister->mutatingIncrement($slug);
+
+        return $slug;
     }
 
     /** @param \Closure(string):string $slugCallback */
@@ -51,6 +80,12 @@ final class HeaderSlug implements Configurable
         return new self(true, $slugCallback);
     }
 
+    /** @param \Closure(string,int):string $duplicationCallback */
+    public static function withDuplicationCallback($duplicationCallback): self
+    {
+        return new self(true, null, $duplicationCallback);
+    }
+
     /** @return self */
     public static function enabled()
     {
diff --git a/src/Configurables/SlugRegister.php b/src/Configurables/SlugRegister.php
new file mode 100644
index 0000000..1a659e0
--- /dev/null
+++ b/src/Configurables/SlugRegister.php
@@ -0,0 +1,44 @@
+<?php
+
+namespace Erusev\Parsedown\Configurables;
+
+use Erusev\Parsedown\MutableConfigurable;
+
+final class SlugRegister implements MutableConfigurable
+{
+    /** @var array<string, int> */
+    private $register;
+
+    /**
+     * @param array<string, int> $register
+     */
+    public function __construct(array $register = [])
+    {
+        $this->register = $register;
+    }
+
+    /** @return self */
+    public static function initial()
+    {
+        return new self;
+    }
+
+    public function mutatingIncrement(string $slug): int
+    {
+        if (! isset($this->register[$slug])) {
+            $this->register[$slug] = 0;
+        }
+
+        return ++$this->register[$slug];
+    }
+
+    public function slugCount(string $slug): int
+    {
+        return $this->register[$slug] ?? 0;
+    }
+
+    public function isolatedCopy(): self
+    {
+        return new self($this->register);
+    }
+}
diff --git a/tests/data/slug_heading.html b/tests/data/slug_heading.html
index d379bb4..56b6021 100644
--- a/tests/data/slug_heading.html
+++ b/tests/data/slug_heading.html
@@ -1,10 +1,11 @@
 <h1 id="foo">foo</h1>
 <h1 id="foo-bar">foo bar</h1>
 <h1 id="foobar">foo_bar</h1>
-<h1 id="foobar">foo+bar</h1>
+<h1 id="foobar-1">foo+bar-1</h1>
+<h1 id="foobar-2">foo+bar</h1>
 <h1 id="2rer0ගම්මැද්ද-v-force-ඉනොවේශන්-නේෂන්-සඳහා-එවූ-නි">2rer*(0👍ගම්මැද්ද V FORCE ඉනොවේශන් නේෂන් සඳහා එවූ නි</h1>
-<h2 id="foo">foo</h2>
-<h2 id="foo-bar">foo bar</h2>
-<h2 id="foobar">foo_bar</h2>
-<h2 id="foobar">foo+bar</h2>
-<h2 id="2rer0ගම්මැද්ද-v-force-ඉනොවේශන්-නේෂන්-සඳහා-එවූ-නි">2rer*(0👍ගම්මැද්ද V FORCE ඉනොවේශන් නේෂන් සඳහා එවූ නි</h2>
\ No newline at end of file
+<h2 id="foo-1">foo</h2>
+<h2 id="foo-bar-1">foo bar</h2>
+<h2 id="foobar-3">foo_bar</h2>
+<h2 id="foobar-4">foo+bar</h2>
+<h2 id="2rer0ගම්මැද්ද-v-force-ඉනොවේශන්-නේෂන්-සඳහා-එවූ-නි-1">2rer*(0👍ගම්මැද්ද V FORCE ඉනොවේශන් නේෂන් සඳහා එවූ නි</h2>
\ No newline at end of file
diff --git a/tests/data/slug_heading.md b/tests/data/slug_heading.md
index 01f66d6..2317bb5 100644
--- a/tests/data/slug_heading.md
+++ b/tests/data/slug_heading.md
@@ -4,6 +4,8 @@
 
 # foo_bar
 
+# foo+bar-1
+
 # foo+bar
 
 # 2rer*(0👍ගම්මැද්ද V FORCE ඉනොවේශන් නේෂන් සඳහා එවූ නි
diff --git a/tests/src/Configurables/HeaderSlugTest.php b/tests/src/Configurables/HeaderSlugTest.php
index 7b4806a..e831a70 100644
--- a/tests/src/Configurables/HeaderSlugTest.php
+++ b/tests/src/Configurables/HeaderSlugTest.php
@@ -3,6 +3,7 @@
 namespace Erusev\Parsedown\Tests\Configurables;
 
 use Erusev\Parsedown\Configurables\HeaderSlug;
+use Erusev\Parsedown\Configurables\SlugRegister;
 use Erusev\Parsedown\State;
 use PHPUnit\Framework\TestCase;
 
@@ -19,6 +20,7 @@ final class HeaderSlugTest extends TestCase
 
         $this->assertSame(true, $State->get(HeaderSlug::class)->isEnabled());
     }
+
     /**
      * @return void
      * @throws \PHPUnit\Framework\ExpectationFailedException
@@ -32,7 +34,27 @@ final class HeaderSlugTest extends TestCase
 
         $this->assertSame(
             'foo_bar',
-            $HeaderSlug->transform('foo bar')
+            $HeaderSlug->transform(SlugRegister::initial(), 'foo bar')
+        );
+    }
+
+    /**
+     * @return void
+     * @throws \PHPUnit\Framework\ExpectationFailedException
+     * @throws \SebastianBergmann\RecursionContext\InvalidArgumentException
+     */
+    public function testCustomDuplicationCallback()
+    {
+        $HeaderSlug = HeaderSlug::withDuplicationCallback(function (string $t, int $n): string {
+            return $t . '_' . \strval($n-1);
+        });
+
+        $SlugRegister = new SlugRegister;
+        $HeaderSlug->transform($SlugRegister, 'foo bar');
+
+        $this->assertSame(
+            'foo-bar_1',
+            $HeaderSlug->transform($SlugRegister, 'foo bar')
         );
     }
 }