Why mysql INSERT ... ON DUPLICATE KEY UPDATE can break RBR replication on a master / master configuration

here is the problem:

• 2 MySQL 5.5 servers
• Row based replications + master master
• Writes are on both servers (both active)
• autoinc trick (1 server odd, the other one even)

I have a table like

byUserDailyStatistics:

• id (PK + AUTO INC)
• date
• idUser
• metric1
• metric2
• UNIQUE(idUser, date)

All requests are

INSERT INTO byEmailDailyStatistics
(date, idUser, metric1, metric2)
VALUES (:date, :user:, 1, 1)
ON DUPLICATE KEY UPDATE
metric1 = metric1 + 1,
metric2 = metric2 +1

And sometimes, the replication breaks with message like

could not execute Write_rows event on table stats.byUserDailyStatistics; Duplicate entry '6447412-2016-01-06' for key 'UNIQUE', Error_code: 1062; handler error HA_ERR_FOUND_DUPP_KEY; the event's master log mysql-bin.035580, end_log_pos 279798813

What could be the origin of this issue?