diff --git a/lib/Command/CreateAccount.php b/lib/Command/CreateAccount.php
index aa515ca448..e2080c84ac 100644
--- a/lib/Command/CreateAccount.php
+++ b/lib/Command/CreateAccount.php
@@ -97,6 +97,11 @@ protected function execute(InputInterface $input, OutputInterface $output): int
 			return 1;
 		}
 
+		if (empty($this->accountService->findByUserIdAndAddress($userId, $email)) === false) {
+			$output->writeln("<error>User $userId already has an account for $email</error>");
+			return 1;
+		}
+
 		$account = new MailAccount();
 		$account->setUserId($userId);
 		$account->setName($name);
diff --git a/lib/Controller/AccountsController.php b/lib/Controller/AccountsController.php
index 81cb19067f..393da826bf 100644
--- a/lib/Controller/AccountsController.php
+++ b/lib/Controller/AccountsController.php
@@ -338,6 +338,12 @@ public function create(string $accountName,
 			$this->logger->info('Creating account disabled by admin.');
 			return MailJsonResponse::error('Could not create account');
 		}
+		if (empty($this->accountService->findByUserIdAndAddress($this->userId, $emailAddress)) === false) {
+			$this->logger->info('Trying to create already existing account.');
+			return MailJsonResponse::fail([
+				'error' => 'ACCOUNT_EXISTS',
+			]);
+		}
 		if (!$this->hostValidator->isValid($imapHost)) {
 			$this->logger->debug('Prevented access to invalid IMAP host', [
 				'host' => $imapHost,
diff --git a/src/components/AccountForm.vue b/src/components/AccountForm.vue
index 217a214c1f..ec5b9c566d 100644
--- a/src/components/AccountForm.vue
+++ b/src/components/AccountForm.vue
@@ -692,6 +692,8 @@ export default {
 					} else if (error.data.service === 'SMTP') {
 						this.feedback = t('mail', 'SMTP server is not reachable')
 					}
+				} else if (error.data?.error === 'ACCOUNT_EXISTS') {
+					this.feedback = t('mail', 'Account already exists')
 				} else if (error.data?.error === 'AUTHENTICATION_WRONG_PASSWORD') {
 					if (error.data.service === 'IMAP') {
 						this.feedback = t('mail', 'IMAP username or password is wrong')
diff --git a/tests/Unit/Controller/AccountsControllerTest.php b/tests/Unit/Controller/AccountsControllerTest.php
index d11ebe2603..e2b0bc938b 100644
--- a/tests/Unit/Controller/AccountsControllerTest.php
+++ b/tests/Unit/Controller/AccountsControllerTest.php
@@ -299,6 +299,30 @@ public function testCreateManualNotAllowed(): void {
 		self::assertEquals($expectedResponse, $response);
 	}
 
+	public function testCreateManualNoDuplicate(): void {
+		$email = 'user@domain.tld';
+		$accountName = 'Mail';
+		$imapHost = 'localhost';
+		$imapPort = 993;
+		$imapSslMode = 'ssl';
+		$imapUser = 'user@domain.tld';
+		$imapPassword = 'mypassword';
+		$smtpHost = 'localhost';
+		$smtpPort = 465;
+		$smtpSslMode = 'none';
+		$smtpUser = 'user@domain.tld';
+		$smtpPassword = 'mypassword';
+		$this->accountService->expects(self::once())
+			->method('findByUserIdAndAddress')
+			->willReturn(['existing account']);
+		$this->setupService->expects(self::never())
+			->method('createNewAccount');
+
+		$expectedResponse = \OCA\Mail\Http\JsonResponse::fail(['error' => 'ACCOUNT_EXISTS']);
+		$response = $this->controller->create($accountName, $email, $imapHost, $imapPort, $imapSslMode, $imapUser, $smtpHost, $smtpPort, $smtpSslMode, $smtpUser, $imapPassword, $smtpPassword);
+
+		self::assertEquals($expectedResponse, $response);
+	}
 
 	public function testCreateManualFailure(): void {
 		$email = 'user@domain.tld';