diff --git a/app/Http/Requests/RegisterRequest.php b/app/Http/Requests/RegisterRequest.php
index ad09dd436..c776caca3 100644
--- a/app/Http/Requests/RegisterRequest.php
+++ b/app/Http/Requests/RegisterRequest.php
@@ -2,6 +2,7 @@
 
 namespace App\Http\Requests;
 
+use App\Rules\UniqueGitHubUser;
 use Illuminate\Foundation\Http\FormRequest;
 
 class RegisterRequest extends FormRequest
@@ -19,7 +20,7 @@ public function rules()
             'username' => 'required|alpha_dash|max:40|unique:users',
             'rules' => 'accepted',
             'terms' => 'accepted',
-            'github_id' => 'required',
+            'github_id' => ['required', new UniqueGitHubUser],
         ];
     }
 
diff --git a/app/Rules/UniqueGitHubUser.php b/app/Rules/UniqueGitHubUser.php
new file mode 100644
index 000000000..e326d2d65
--- /dev/null
+++ b/app/Rules/UniqueGitHubUser.php
@@ -0,0 +1,34 @@
+<?php
+
+namespace App\Rules;
+
+use App\Concerns\SendsAlerts;
+use App\Models\User;
+use Illuminate\Contracts\Validation\Rule;
+use Illuminate\Database\Eloquent\ModelNotFoundException;
+
+final class UniqueGitHubUser implements Rule
+{
+    use SendsAlerts;
+
+    private User $user;
+
+    public function passes($attribute, $value): bool
+    {
+        try {
+            $this->user = User::findByGithubId($value);
+        } catch (ModelNotFoundException) {
+            return true;
+        }
+
+        return false;
+    }
+
+    public function message()
+    {
+        $this->error('errors.github_account_exists', [
+            'username' => '@'.$this->user->githubUsername(),
+            'login' => route('login'),
+        ]);
+    }
+}
diff --git a/resources/lang/en/errors.php b/resources/lang/en/errors.php
index 6d2dc3c01..55c8844e8 100644
--- a/resources/lang/en/errors.php
+++ b/resources/lang/en/errors.php
@@ -5,4 +5,5 @@
     'fields' => 'Something went wrong. Please review the fields below.',
     'github_invalid_state' => 'The request timed out. Please try again.',
     'github_account_too_young' => 'Your Github account needs to be older than 2 weeks in order to register.',
+    'github_account_exists' => 'We already found a user with the given GitHub account (:username). Would you like to <a href=":login">login</a> instead?',
 ];
diff --git a/tests/Feature/AuthTest.php b/tests/Feature/AuthTest.php
index 44dc9a1dd..d70e5296d 100644
--- a/tests/Feature/AuthTest.php
+++ b/tests/Feature/AuthTest.php
@@ -1,5 +1,6 @@
 <?php
 
+use App\Models\User;
 use Carbon\Carbon;
 use Illuminate\Auth\Events\Registered;
 use Illuminate\Contracts\Auth\PasswordBroker;
@@ -61,6 +62,24 @@
         ->see('The username must only contain letters, numbers, dashes and underscores.');
 });
 
+test('registration fails with a duplicate github id', function () {
+    User::factory()->create(['github_id' => 123, 'github_username' => 'johndoe']);
+
+    session(['githubData' => ['id' => 123, 'username' => 'johndoe']]);
+
+    $this->visit('/register')
+        ->type('John Doe', 'name')
+        ->type('john.doe@example.com', 'email')
+        ->type('johndoe', 'username')
+        ->type('123', 'github_id')
+        ->type('johndoe', 'github_username')
+        ->check('rules')
+        ->check('terms')
+        ->press('Register')
+        ->seePageIs('/register')
+        ->see('We already found a user with the given GitHub account (@johndoe). Would you like to <a href="http://localhost/login">login</a> instead?');
+});
+
 test('users can resend the email verification', function () {
     $this->login(['email_verified_at' => null]);