The target replication core's ID is the same as the source replication core.
This may happen if you used the same RASR USB key from the source replication core to factory reset / rebuild the target replication core.
This may also happen if you have "cloned" a source core that happens to be virtual to be used as a new target replication core.
Use the "graceful shutdown" approach to stop the core service at the target replication core you want to establish replication with.
Use regedit to find the following registry key - HKEY_LOCAL_MACHINE\SOFTWARE\AppRecovery\Core\CoreId
Export (save) the key for safe keeping, just in case.
Delete the key.
Start the core service you stopped in step 1.
Establish replication from the source replication core.