diff --git a/models/DependencyModel.php b/models/DependencyModel.php
index a7caf31..8f37899 100644
--- a/models/DependencyModel.php
+++ b/models/DependencyModel.php
@@ -172,6 +172,27 @@ class DependencyModel
         }
         $checkStmt->close();
 
+        // Also check the semantic inverse: "A blocks B" and "B blocked_by A"
+        // describe the same relationship, so adding one from either ticket's
+        // page must be rejected as a duplicate of the other. relates_to is
+        // its own inverse (symmetric); duplicates has no defined inverse type.
+        $inverseTypes = ['blocks' => 'blocked_by', 'blocked_by' => 'blocks', 'relates_to' => 'relates_to'];
+        if (isset($inverseTypes[$type])) {
+            $inverseType = $inverseTypes[$type];
+            $checkInverseSql = "SELECT dependency_id FROM ticket_dependencies
+                                 WHERE ticket_id = ? AND depends_on_id = ? AND dependency_type = ?";
+            $checkInverseStmt = $this->conn->prepare($checkInverseSql);
+            $checkInverseStmt->bind_param("sss", $dependsOnId, $ticketId, $inverseType);
+            $checkInverseStmt->execute();
+            $inverseResult = $checkInverseStmt->get_result();
+
+            if ($inverseResult->num_rows > 0) {
+                $checkInverseStmt->close();
+                return ['success' => false, 'error' => 'This relationship already exists'];
+            }
+            $checkInverseStmt->close();
+        }
+
         // Check for circular dependency
         if ($this->wouldCreateCycle($ticketId, $dependsOnId, $type)) {
             return ['success' => false, 'error' => 'This would create a circular dependency'];