Magento doule decrementing stock when shipping orders

double_decrementing_stock_on_shipment.patch

--- Model/GetItemsToDeductFromShipment.php	2020-11-26 10:23:56.827000000 +0000
+++ Model/GetItemsToDeductFromShipment.php	2020-11-26 10:22:50.741302876 +0000
@@ -17,6 +17,7 @@
 use Magento\Sales\Model\Order\Item as OrderItem;
 use Magento\Sales\Model\Order\Shipment;
 use Magento\Sales\Model\Order\Shipment\Item;
+use Magento\CatalogInventory\Api\StockConfigurationInterface;
 
 /**
  * Get source items for deduction class.
@@ -44,6 +45,11 @@
     private $itemCouldBeDeducted;
 
     /**
+     * @var StockConfigurationInterface
+     */
+    protected $stockConfiguration;
+
+    /**
      * @param GetSkuFromOrderItemInterface $getSkuFromOrderItem
      * @param Json $jsonSerializer
      * @param ItemToDeductInterfaceFactory $itemToDeduct
@@ -53,6 +59,7 @@
         GetSkuFromOrderItemInterface $getSkuFromOrderItem,
         Json $jsonSerializer,
         ItemToDeductInterfaceFactory $itemToDeduct,
+        StockConfigurationInterface $stockConfiguration,
         IsItemCouldBeDeductedByTypes $itemCouldBeDeducted = null
     ) {
         $this->jsonSerializer = $jsonSerializer;
@@ -60,6 +67,7 @@
         $this->getSkuFromOrderItem = $getSkuFromOrderItem;
         $this->itemCouldBeDeducted = $itemCouldBeDeducted ?: ObjectManager::getInstance()
             ->get(IsItemCouldBeDeductedByTypes::class);
+        $this->stockConfiguration = $stockConfiguration;
     }
 
     /**
@@ -73,6 +81,13 @@
     {
         $itemsToShip = [];
 
+        // if canSubtractQty === true
+        // this means "deduct when order is placed" so we
+        // don't want to deduct it again when shipping the order
+        if($this->stockConfiguration->canSubtractQty($shipment->getStoreId())){
+            return $itemsToShip;
+        }
+
         /** @var \Magento\Sales\Model\Order\Shipment\Item $shipmentItem */
         foreach ($shipment->getAllItems() as $shipmentItem) {
             $orderItem = $shipmentItem->getOrderItem();

example_composer.json

## this should be added to your composer.json. It should NOT replace your composer.json	
"extra": {	
   "patches": {	
     "magento/module-inventory-shipping": {	
          "double decrementing stock issue": "patches/double_decrementing_stock_on_shipment.patch"	
      }	
   }	
}