When crafting a custom shield, I tend to put more than 1 banner in at a time. When you put more than one in, it dupes everything and puts the new banners in the hotbar, and the old ones remain in the crafting table.

I put in 16 banners and 1 shield, when I accept the craft, 16 banners and the custom shield enter my hotbar, and 15 are left in the crafting table, both creative and survival.