Are both of them ula addresses? (both of them starting with 2). If they are not the same then the ISP is providing an internal unique address for isp internal configurations.

If so, are they having the same network? ( The first 48 bits) if not, then is probably a miss configuration but probably in their side. But with no practical effects. You could ignore it.

If yes again, then it is a miss configuration and it shouldn't happen, but this time it could be in your side, check that the dhcpv6 daemon doesn't try to give an ipv6 address To your Wan port