Overlapping IPV6 address on Router in packet tracer

Question

Below is the command I am giving to configure my serial0/0/0 on my PT router.

HQ(config)#int Serial0/0/0

HQ(config-if)#ipv6 address 2001:DB8:ACAD::20:2/64

%Serial0/0/0: Error: 2001:DB8:ACAD::/64 is overlapping with 2001:DB8:ACAD::/64 
on GigabitEthernet0/1

My ipv6 interfaces are showing the following BEFORE trying to change serial 0/0/0:

HQ#show ipv6 int brief

GigabitEthernet0/0         [administratively down/down]
GigabitEthernet0/1         [administratively down/down]
    FE80::1
    2001:DB8:ACAD::1
Serial0/0/0                [up/up]
    FE80::203:E4FF:FE07:5201
    2001:DB8:ACAD:20::2
Serial0/0/1                [up/up]
    FE80::203:E4FF:FE07:5201
    2001:DB8:ACAD:2::3

In the lab, the serial connection in the configuration is incorrectly set, on purpose. I'm supposed to run these commands to figure out the problem. When I try to correct serial with the correct information in the addressing table, which is correct, I get the error at the beginning of the post. Serial 0/0/0 should be configured with

Here is the way these interfaces are supposed to be configured:

HQ
G0/1
192.168.0.1/25
2001:DB8:ACAD::1/64
FE80::1 link-local
N/A

S0/0/0 (DCE)
10.1.1.2/30
2001:DB8:ACAD::20:2/64
N/A

S0/0/1
192.168.0.253/30
2001:DB8:ACAD:2::1/30
N/A

Help!!

Solution

I think either your homework problem has a typo or you made a typo when you copied it into your question here.

These two IPv6 networks overlap:

2001:DB8:ACAD::1/64  
2001:DB8:ACAD::20:2/64  

I'm wondering if someone put the double-colon in the wrong place, because these two IPv6 networks do not overlap:

2001:DB8:ACAD::1/64  
2001:DB8:ACAD:20::2/64  

Carefully double-check your assignment to see where the double-colons are, double-check the prefix lengths, and double-check the prefixes themselves.