I tried to configure Ipv6 static routing on three routers, I configured ipv6 addresses on all three routers using ipv6 address auto config, The middle router have been configured with different global unicast addresses perfectly on it's both serial interfaces, but I saw those serial interfaces have been configured with same link local addresses, I'm a bit confused, can someone please answer!
Link local addresses have, as the name implies, only local relevance on the link. Every link uses the same link local address block, and duplicate addresses on different links are no problem. They only have to be unique on each link.
Interface identifiers (the rightmost bits of an address) for link local addresses are often generated based on the MAC address of the interface. Because serial interfaces don't have a MAC address the router has to use something else that is likely to be unique. It might for example take a random number, the MAC address of a different interface, it's serial number etc. So if it has multiple interfaces that don't have their own MAC address it is perfectly normal to reuse the same interface identifier on all of them.