The bug
By placing an illager banner and a white banner in the crafting grid, I was able to create a second illager banner.
I don't think this is working as intended, because illager banners are otherwise uncraftable in survival. They have seven patterns, while only six are possible in survival without cheats. I had assumed they were intended to be obtainable solely as trophies from illager patrols.
How to reproduce
Place an illager banner and a white banner in the crafting grid.
Comments 8
I too think they shouldn't be craftable - they seem to be a unique trophy for defeating the pillager patrol.
Urielsalis, the intent seems to be for them not to be obtainable in survival except as a drop from patrol leaders. By giving them seven patterns, the devs assured they could not be crafted; it seems likely that leaving the copying mechanism in was an oversight.
There's one bit of further evidence: The Illager Banner has a unique colored title, different from every other banner in the game. That led me to think it's meant to be a unique item.
Why do you expect only those banners to not be copyable?