diff --git a/config/default/core.entity_form_display.node.post.default.yml b/config/default/core.entity_form_display.node.post.default.yml
index cae82ea..99bc3fc 100644
--- a/config/default/core.entity_form_display.node.post.default.yml
+++ b/config/default/core.entity_form_display.node.post.default.yml
@@ -7,6 +7,7 @@ dependencies:
     - field.field.node.post.field_excerpt
     - field.field.node.post.field_has_tweet
     - field.field.node.post.field_images
+    - field.field.node.post.field_sent_to_social_media
     - field.field.node.post.field_series
     - field.field.node.post.field_tags
     - node.type.post
@@ -127,4 +128,5 @@ content:
     region: content
     settings: {  }
     third_party_settings: {  }
-hidden: {  }
+hidden:
+  field_sent_to_social_media: true
diff --git a/config/default/core.entity_view_display.node.post.default.yml b/config/default/core.entity_view_display.node.post.default.yml
index 4aa2564..bad63da 100644
--- a/config/default/core.entity_view_display.node.post.default.yml
+++ b/config/default/core.entity_view_display.node.post.default.yml
@@ -7,6 +7,7 @@ dependencies:
     - field.field.node.post.field_excerpt
     - field.field.node.post.field_has_tweet
     - field.field.node.post.field_images
+    - field.field.node.post.field_sent_to_social_media
     - field.field.node.post.field_series
     - field.field.node.post.field_tags
     - node.type.post
@@ -25,6 +26,16 @@ content:
     settings: {  }
     third_party_settings: {  }
     region: content
+  field_sent_to_social_media:
+    weight: 3
+    label: above
+    settings:
+      format: default
+      format_custom_false: ''
+      format_custom_true: ''
+    third_party_settings: {  }
+    type: boolean
+    region: content
   field_tags:
     weight: 2
     label: above
diff --git a/config/default/core.entity_view_display.node.post.teaser.yml b/config/default/core.entity_view_display.node.post.teaser.yml
index dfd5af7..6c4f01b 100644
--- a/config/default/core.entity_view_display.node.post.teaser.yml
+++ b/config/default/core.entity_view_display.node.post.teaser.yml
@@ -8,6 +8,7 @@ dependencies:
     - field.field.node.post.field_excerpt
     - field.field.node.post.field_has_tweet
     - field.field.node.post.field_images
+    - field.field.node.post.field_sent_to_social_media
     - field.field.node.post.field_series
     - field.field.node.post.field_tags
     - node.type.post
@@ -34,5 +35,6 @@ hidden:
   body: true
   field_has_tweet: true
   field_images: true
+  field_sent_to_social_media: true
   field_series: true
   field_tags: true
diff --git a/config/default/field.field.node.post.field_sent_to_social_media.yml b/config/default/field.field.node.post.field_sent_to_social_media.yml
new file mode 100644
index 0000000..99c5811
--- /dev/null
+++ b/config/default/field.field.node.post.field_sent_to_social_media.yml
@@ -0,0 +1,23 @@
+uuid: 4f639dd0-6634-43ea-89a8-141dfde87059
+langcode: en
+status: true
+dependencies:
+  config:
+    - field.storage.node.field_sent_to_social_media
+    - node.type.post
+id: node.post.field_sent_to_social_media
+field_name: field_sent_to_social_media
+entity_type: node
+bundle: post
+label: 'Sent to social media'
+description: ''
+required: false
+translatable: false
+default_value:
+  -
+    value: 0
+default_value_callback: ''
+settings:
+  on_label: 'On'
+  off_label: 'Off'
+field_type: boolean
diff --git a/config/default/field.storage.node.field_sent_to_social_media.yml b/config/default/field.storage.node.field_sent_to_social_media.yml
new file mode 100644
index 0000000..ec314a3
--- /dev/null
+++ b/config/default/field.storage.node.field_sent_to_social_media.yml
@@ -0,0 +1,18 @@
+uuid: 2fead662-4d91-4788-b739-8e1a56d236af
+langcode: en
+status: true
+dependencies:
+  module:
+    - node
+id: node.field_sent_to_social_media
+field_name: field_sent_to_social_media
+entity_type: node
+type: boolean
+settings: {  }
+module: core
+locked: false
+cardinality: 1
+translatable: true
+indexes: {  }
+persist_with_no_fields: false
+custom_storage: false
diff --git a/web/modules/custom/custom/custom.install b/web/modules/custom/custom/custom.install
new file mode 100644
index 0000000..7965eb7
--- /dev/null
+++ b/web/modules/custom/custom/custom.install
@@ -0,0 +1,17 @@
+<?php
+
+declare(strict_types=1);
+
+use Drupal\custom\Repository\PostRepository;
+
+/**
+ * Mark existing blog posts as sent to social media.
+ */
+function custom_update_8001(): void {
+  $posts = \Drupal::service(PostRepository::class)->getAll();
+
+  foreach ($posts as $post) {
+    $post->set('field_sent_to_social_media', TRUE);
+    $post->save();
+  }
+}
diff --git a/web/modules/custom/custom/custom.services.yml b/web/modules/custom/custom/custom.services.yml
index c553e28..21c4bb3 100644
--- a/web/modules/custom/custom/custom.services.yml
+++ b/web/modules/custom/custom/custom.services.yml
@@ -7,5 +7,8 @@ services:
     tags:
       - { name: event_subscriber }
 
+  Drupal\custom\Repository\PostRepository:
+    autowire: true
+
   Drupal\custom\Service\TalkCounter:
     autowire: true
diff --git a/web/modules/custom/custom/src/Entity/Node/Post.php b/web/modules/custom/custom/src/Entity/Node/Post.php
index 30427a3..4f33f9b 100644
--- a/web/modules/custom/custom/src/Entity/Node/Post.php
+++ b/web/modules/custom/custom/src/Entity/Node/Post.php
@@ -18,6 +18,10 @@ use Drupal\node\Entity\Node;
  */
 class Post extends Node implements ContentEntityBundleInterface {
 
+  public function hasBeenSentToSocialMedia(): bool {
+    return (bool) $this->get('field_sent_to_social_media')->getString();
+  }
+
   public function hasTweet(): bool {
     return (bool) $this->get('field_has_tweet')->getString();
   }
diff --git a/web/modules/custom/custom/src/EventSubscriber/PushBlogPostToSocialMedia.php b/web/modules/custom/custom/src/EventSubscriber/PushBlogPostToSocialMedia.php
index 89e72fc..ae1e908 100644
--- a/web/modules/custom/custom/src/EventSubscriber/PushBlogPostToSocialMedia.php
+++ b/web/modules/custom/custom/src/EventSubscriber/PushBlogPostToSocialMedia.php
@@ -37,7 +37,10 @@ final class PushBlogPostToSocialMedia implements EventSubscriberInterface {
       return;
     }
 
-    // TODO: Check that the post has not already been pushed.
+    // If this post has already been sent to social media, do not send it again.
+    if ($entity->hasBeenSentToSocialMedia()) {
+      return;
+    }
 
     $url = \Drupal::configFactory()->get('opdavies_talks.config')
       ->get('zapier_post_tweet_url');
@@ -47,6 +50,8 @@ final class PushBlogPostToSocialMedia implements EventSubscriberInterface {
         'message' => $entity->toTweet(),
       ],
     ]);
+
+    $entity->set('field_sent_to_social_media', TRUE);
   }
 
 }
diff --git a/web/modules/custom/custom/src/Repository/PostRepository.php b/web/modules/custom/custom/src/Repository/PostRepository.php
new file mode 100644
index 0000000..809c9ae
--- /dev/null
+++ b/web/modules/custom/custom/src/Repository/PostRepository.php
@@ -0,0 +1,27 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\custom\Repository;
+
+use Drupal\Core\Entity\EntityStorageInterface;
+use Drupal\Core\Entity\EntityTypeManagerInterface;
+use Illuminate\Support\Collection;
+
+final class PostRepository {
+
+  private EntityStorageInterface $nodeStorage;
+
+  public function __construct(EntityTypeManagerInterface $entityTypeManager) {
+    $this->nodeStorage = $entityTypeManager->getStorage('node');
+  }
+
+  public function getAll(): Collection {
+    return new Collection(
+      $this->nodeStorage->loadByProperties([
+        'type' => 'post',
+      ])
+    );
+  }
+
+}