A link-local address is only used on a single link. Traffic on a link-local network is confined to that network and cannot be routed to a different network. All IPv6 interfaces use the same link-local network, so addresses on one link-local network can be the same on a different link-local network.
See RFC 4291, IP Version 6 Addressing Architecture:
2.5.6. Link-Local IPv6 Unicast Addresses Link-Local addresses are for use on a single link. Link-Local addresses have the following format:
| 10 | | bits | 54 bits | 64 bits | +----------+-------------------------+----------------------------+ |1111111010| 0 | interface ID | +----------+-------------------------+----------------------------+Link-Local addresses are designed to be used for addressing on a single link for purposes such as automatic address configuration, neighbor discovery, or when no routers are present. Routers must not forward any packets with Link-Local source or destination addresses to other links.