diff --git a/inc/class-job.php b/inc/class-job.php
index f6087b5..58f259e 100644
--- a/inc/class-job.php
+++ b/inc/class-job.php
@@ -65,7 +65,13 @@ public function save() {
 			$result = $wpdb->update( $this->get_table(), $data, $where, $this->row_format( $data ), $this->row_format( $where ) );
 		} else {
 			$result = $wpdb->insert( $this->get_table(), $data, $this->row_format( $data ) );
-			$this->id = $wpdb->insert_id;
+
+			// Swallow duplicate insert errors.
+			if ( ! $result && strpos( $wpdb->last_error, '[1062]' ) !== false ) {
+				$result = true;
+			} else {
+				$this->id = $wpdb->insert_id;
+			}
 		}
 
 		self::flush_query_cache();
diff --git a/inc/namespace.php b/inc/namespace.php
index 3288229..5fd9b63 100644
--- a/inc/namespace.php
+++ b/inc/namespace.php
@@ -90,8 +90,14 @@ function create_tables() {
 		`interval` int unsigned DEFAULT NULL,
 		`status` varchar(255) NOT NULL DEFAULT 'waiting',
 		`schedule` varchar(255) DEFAULT NULL,
+		`hash` binary(32) GENERATED ALWAYS AS (
+			UNHEX(SHA2(CONCAT_WS(
+				'-', `site`, `hook`, `args`, `nextrun`, `status`, `schedule`
+			), 256))
+		) STORED,
 
 		PRIMARY KEY (`id`),
+		UNIQUE INDEX `uniq` (`hash`),
 		KEY `status` (`status`),
 		KEY `site` (`site`),
 		KEY `hook` (`hook`)
diff --git a/inc/upgrade/namespace.php b/inc/upgrade/namespace.php
index 853e97e..b2ffedb 100644
--- a/inc/upgrade/namespace.php
+++ b/inc/upgrade/namespace.php
@@ -8,6 +8,7 @@
 use const HM\Cavalcade\Plugin\DATABASE_VERSION;
 use HM\Cavalcade\Plugin as Cavalcade;
 use HM\Cavalcade\Plugin\Job;
+use WP_CLI;
 
 /**
  * Update the Cavalcade database version if required.
@@ -37,6 +38,10 @@ function upgrade_database() {
 		upgrade_database_4();
 	}
 
+	if ( $database_version < 5 ) {
+		upgrade_database_5();
+	}
+
 	update_site_option( 'cavalcade_db_version', DATABASE_VERSION );
 
 	Job::flush_query_cache();
@@ -100,3 +105,47 @@ function upgrade_database_4() {
 
 	$wpdb->query( $query );
 }
+
+/**
+ * Upgrade Cavalcade database tables to version 5.
+ *
+ * Add unique index to prevent duplicate entries being created.
+ */
+function upgrade_database_5() {
+	global $wpdb;
+
+	$queries = [
+		// Add generated stored hash column.
+		"ALTER TABLE `{$wpdb->base_prefix}cavalcade_jobs`
+			ADD COLUMN `hash` BINARY(32) GENERATED ALWAYS AS (
+			  	UNHEX(SHA2(CONCAT_WS(
+					'-', `site`, `hook`, `args`, `nextrun`, `status`, `schedule`
+				), 256))
+			) STORED;",
+		// Remove full group by mode requirement if set.
+		"SET @sql_mode_tmp = (SELECT @@sql_mode);",
+		"SET sql_mode = (SELECT REPLACE(@@sql_mode,'ONLY_FULL_GROUP_BY',''));",
+		// Remove any current duplicates.
+		"DELETE t1 FROM `{$wpdb->base_prefix}cavalcade_jobs` t1
+			INNER JOIN (
+				SELECT MIN(`id`) as `id`, `hash`
+				FROM `{$wpdb->base_prefix}cavalcade_jobs`
+				GROUP BY `hash`
+				HAVING COUNT(*) > 1
+			) t2
+			ON t1.`hash` = t2.`hash`
+				AND t1.id != t2.id;",
+		// Add a unique index on the hash column.
+		"ALTER TABLE `{$wpdb->base_prefix}cavalcade_jobs` ADD UNIQUE INDEX `uniq` (`hash`);",
+		// Restore the SQL mode.
+		"SET sql_mode = (SELECT @sql_mode_tmp);",
+	];
+
+	foreach ( $queries as $query ) {
+		$wpdb->query( $query );
+
+		if ( defined( 'WP_CLI' ) && WP_CLI && ! empty( $wpdb->last_error ) ) {
+			WP_CLI::error( $wpdb->last_error, false );
+		}
+	}
+}
diff --git a/plugin.php b/plugin.php
index 7ebf88c..258f4e6 100644
--- a/plugin.php
+++ b/plugin.php
@@ -12,7 +12,7 @@
 namespace HM\Cavalcade\Plugin;
 
 const DATE_FORMAT = 'Y-m-d H:i:s';
-const DATABASE_VERSION = 4;
+const DATABASE_VERSION = 5;
 
 require __DIR__ . '/inc/namespace.php';
 require __DIR__ . '/inc/class-job.php';