IF a replica was set the server_id on the same value as the master
set global server_id = master_server_id;

it have an immediate effect without warning during running replication.

Only after a restart the well known message occured.

[ERROR] Slave I/O: Fatal error: The slave I/O thread stops because master and slave have equal MariaDB server ids;

An error should be thrown immediate. To Client and error log