Error Duplicate entry - Master-Slave replication

[Sree Mal]

I have a Master-Slave replication running well for almost week. Slave is read-only and Master is high-transaction db and its the only source pushing DML statements to Slave. Suddenly, I see a error on slave status:

Last_Error: Error 'Duplicate entry '12345' for key 'Tablebname'' on query. Default database: 'test_me' Query: 'insert into rep_info (rep_name, create_time, update_time, count) values ('12345', '2012-01-04 01:13:12', '2012-01-04 01:13:12', 1)';

I see that the record already exists on slave db, just wondering why would this happen considering slave db is read-only.

Sree.