Have you heard this:
master to master replication replicated its own queries (replicated to another server and
replicated back)? Although both two servers has own server id. For example, master A reads
binlog on master B, and executed, got duplicate key error. master B reads binlog from
master A, and got duplicate key error. (these two queries are different). The condition
is, On master B, there are no application setup to access database, it only replicates
from master A. No updates on server B.
MySQL used v5.037.