Never expect bukkit API to be intuitive

src/main/java/com/gmail/nossr50/listeners/InventoryListener.java

@@ -117,7 +117,7 @@ public class InventoryListener implements Listener {
                 //Profile doesn't exist
                 if(offlineProfile != null) {
                     //Process smelting
-                    offlineProfile.getSmeltingManager().smeltProcessing(event);
+                    offlineProfile.getSmeltingManager().smeltProcessing(event, furnace);
                 }
             }
         }

src/main/java/com/gmail/nossr50/skills/smelting/SmeltingManager.java

@@ -11,8 +11,10 @@ import com.gmail.nossr50.util.Permissions;
 import com.gmail.nossr50.util.random.RandomChanceUtil;
 import com.gmail.nossr50.util.skills.RankUtils;
 import com.gmail.nossr50.util.skills.SkillActivationType;
+import org.bukkit.block.Furnace;
 import org.bukkit.event.inventory.FurnaceBurnEvent;
 import org.bukkit.event.inventory.FurnaceSmeltEvent;
+import org.bukkit.inventory.FurnaceInventory;
 import org.bukkit.inventory.ItemStack;
 import org.jetbrains.annotations.NotNull;
 
@@ -110,13 +112,13 @@ public class SmeltingManager extends SkillManager {
         }
     }
 
-    public void smeltProcessing(@NotNull FurnaceSmeltEvent furnaceSmeltEvent) {
+    public void smeltProcessing(@NotNull FurnaceSmeltEvent furnaceSmeltEvent, @NotNull Furnace furnace) {
         applyXpGain(Smelting.getResourceXp(furnaceSmeltEvent.getSource()), XPGainReason.PVE, XPGainSource.PASSIVE); //Add XP
 
-        processDoubleSmelt(furnaceSmeltEvent);
+        processDoubleSmelt(furnaceSmeltEvent, furnace);
     }
 
-    private void processDoubleSmelt(@NotNull FurnaceSmeltEvent furnaceSmeltEvent) {
+    private void processDoubleSmelt(@NotNull FurnaceSmeltEvent furnaceSmeltEvent, @NotNull Furnace furnace) {
         ItemStack resultItemStack = furnaceSmeltEvent.getResult();
         /*
             doubleSmeltCondition should be equal to the max
@@ -124,7 +126,7 @@ public class SmeltingManager extends SkillManager {
 
         //Process double smelt
         if (Config.getInstance().getDoubleDropsEnabled(PrimarySkillType.SMELTING, resultItemStack.getType())
-                && canDoubleSmeltItemStack(resultItemStack) //Effectively two less than max stack size
+                && canDoubleSmeltItemStack(furnace) //Effectively two less than max stack size
                 && isSecondSmeltSuccessful()) {
 
             ItemStack doubleSmeltStack = resultItemStack.clone(); //TODO: Necessary?
@@ -133,9 +135,15 @@ public class SmeltingManager extends SkillManager {
         }
     }
 
-    private boolean canDoubleSmeltItemStack(@NotNull ItemStack itemStack) {
-        int resultAmount = itemStack.getAmount(); //Amount before double smelt
-        int itemLimit = itemStack.getMaxStackSize();
+    private boolean canDoubleSmeltItemStack(@NotNull Furnace furnace) {
+        FurnaceInventory furnaceInventory = furnace.getInventory();
+        ItemStack furnaceResult = furnaceInventory.getResult();
+
+        if(furnaceResult == null)
+            return false;
+
+        int resultAmount = furnaceResult.getAmount(); //Amount before double smelt
+        int itemLimit = furnaceResult.getMaxStackSize();
         int doubleSmeltCondition = itemLimit - 2; //Don't double smelt if it would cause an illegal stack size
 
         return resultAmount <= doubleSmeltCondition;