diff --git a/src/gui/guiFormSpecMenu.cpp b/src/gui/guiFormSpecMenu.cpp
index 0e662b282..8b6c78838 100644
--- a/src/gui/guiFormSpecMenu.cpp
+++ b/src/gui/guiFormSpecMenu.cpp
@@ -4447,6 +4447,12 @@ bool GUIFormSpecMenu::OnEvent(const SEvent& event)
 				if (m_left_drag_stacks.size() > 1) {
 					// Finalize the left-dragging
 					for (auto &ds : m_left_drag_stacks) {
+						if (ds.first == *m_selected_item) {
+							// This entry is needed to properly calculate the stack sizes.
+							// The stack already exists, hence no further action needed here.
+							continue;
+						}
+
 						// Check how many items we should move to this slot,
 						// it may be less than the full split
 						Inventory *inv_to = m_invmgr->getInventory(ds.first.inventoryloc);
diff --git a/src/gui/guiInventoryList.h b/src/gui/guiInventoryList.h
index 07ca93c3f..8ef63c8bc 100644
--- a/src/gui/guiInventoryList.h
+++ b/src/gui/guiInventoryList.h
@@ -43,7 +43,7 @@ public:
 		{
 		}
 
-		bool operator==(const ItemSpec& other)
+		bool operator==(const ItemSpec& other) const
 		{
 			return inventoryloc == other.inventoryloc &&
 					listname == other.listname && i == other.i;