When you Craft a banner and shield together it will repair the shield and you still keep the original banner.
Linked issues
is duplicated by 5
MC-105673
Putting A Design on a Sheild Fixes the Sheild
Resolved
MC-89707
Shields When Crafted with a banner heals?
Resolved
MC-100770
Shield gets full durability when crafted with a banner
Resolved
MC-101028
repair a shield in a crafting table
Resolved
MC-102815
Damaged Shield + Banner = Repaired Shield
Resolved
keeping the original banner is intended