chocolatecake5000
Terrarian
I don't know if this is the right section but if you put a banner in lava and break it you can dupelicate it. Try it out.
I play pc and I'll go report.Not sure what version do you play but you should report the bug here http://forums.terraria.org/index.php?forums/pc-bug-reports.113/ for PC or here http://forums.terraria.org/index.php?forums/console-bug-reports.138/ for console.