diff --git a/Koha/EDI.pm b/Koha/EDI.pm
index 14cec0cd1c..c003097fa5 100644
--- a/Koha/EDI.pm
+++ b/Koha/EDI.pm
@@ -667,6 +667,32 @@ sub process_quote {
                 my $purchase_order_number = $msg->purchase_order_number;
                 if ($purchase_order_number) {
                     $basket_name = $purchase_order_number;
+
+                    # Check for duplicate purchase order numbers for this vendor (including closed baskets)
+                    my $existing_basket = $schema->resultset('Aqbasket')->search(
+                        {
+                            basketname   => $basket_name,
+                            booksellerid => $quote_message->vendor_id,
+                        }
+                    )->first;
+
+                    if ($existing_basket) {
+
+                        # Log error for duplicate purchase order number but continue processing
+                        my $rff_segment = "RFF+ON:$purchase_order_number";
+                        $quote_message->add_to_edifact_errors(
+                            {
+                                section => $rff_segment,
+                                details => "Duplicate purchase order number '$purchase_order_number' found for vendor "
+                                    . $quote_message->vendor_id . "."
+                            }
+                        );
+                        $logger->warn( "Duplicate purchase order number '$purchase_order_number' for vendor "
+                                . $quote_message->vendor_id
+                                . " (existing basket: "
+                                . $existing_basket->basketno
+                                . ")" );
+                    }
                 }
             }
 
diff --git a/t/db_dependent/Koha/EDI.t b/t/db_dependent/Koha/EDI.t
index 45c3303785..d8e0c32bf3 100755
--- a/t/db_dependent/Koha/EDI.t
+++ b/t/db_dependent/Koha/EDI.t
@@ -38,7 +38,7 @@ my $builder = t::lib::TestBuilder->new;
 my $logger  = t::lib::Mocks::Logger->new();
 
 subtest 'process_quote' => sub {
-    plan tests => 6;
+    plan tests => 7;
 
     $schema->storage->txn_begin;
 
@@ -914,6 +914,67 @@ subtest 'process_quote' => sub {
         $schema->storage->txn_rollback;
     };
 
+    # Test 7: Duplicate purchase order number validation
+    subtest 'duplicate_purchase_order_validation' => sub {
+        plan tests => 5;
+
+        $schema->storage->txn_begin;
+
+        # Create vendor EDI account with po_is_basketname set to true
+        my $account = $builder->build(
+            {
+                source => 'VendorEdiAccount',
+                value  => {
+                    description      => 'test vendor duplicate po',
+                    transport        => 'FILE',
+                    po_is_basketname => 1,
+                },
+            }
+        );
+
+        # Create first basket with purchase order number "orders 23/1" (same as in QUOTES_SMALL.CEQ)
+        my $first_basket = $builder->build(
+            {
+                source => 'Aqbasket',
+                value  => {
+                    basketname   => 'orders 23/1',
+                    booksellerid => $account->{vendor_id},
+                    closedate    => undef,                   # Open basket
+                },
+            }
+        );
+
+        # Use existing test file that contains RFF+ON:orders 23/1
+        my $filename = 'QUOTES_SMALL.CEQ';
+        my $trans    = Koha::Edifact::Transport->new( $account->{id} );
+        $trans->working_directory($dirname);
+
+        my $mhash = $trans->message_hash();
+        $mhash->{message_type} = 'QUOTE';
+        $trans->ingest( $mhash, $filename );
+
+        my $quote = $schema->resultset('EdifactMessage')->find( { filename => $filename } );
+        ok( $quote, 'Quote message created successfully' );
+
+        # Process the quote (this should trigger duplicate detection)
+        process_quote($quote);
+
+        # Check that duplicate purchase order error was logged
+        my $errors = $quote->edifact_errors;
+        ok( $errors->count >= 1, 'At least one error logged during quote processing' );
+
+        # Find the specific duplicate purchase order error
+        my $duplicate_error = $errors->search( { section => 'RFF+ON:orders 23/1' } )->first;
+        ok( $duplicate_error, 'Duplicate purchase order error found' );
+        is( $duplicate_error->section, 'RFF+ON:orders 23/1', 'Error section contains the RFF+ON segment' );
+        like(
+            $duplicate_error->details, qr/Duplicate purchase order number 'orders 23\/1' found for vendor/,
+            'Error details describe the duplicate issue'
+        );
+
+        $schema->storage->txn_rollback;
+    };
+
     # Clean up
     $logger->clear();
     $schema->storage->txn_rollback;