On one MySQL slave server we maintain, we noticed this error
Duplicate entry '2' for key 'PRIMARY'
A quick fix was to issue this command:
SET GLOBAL SQL_SLAVE_SKIP_COUNTER=1; START SLAVE;
Some say that there could be a data entry in the slave server and then it was being written again generating the duplicate error. Some others are of the opinion that if
myisamchk was executed in the master server, it may have removed some entries and tried to create those again. The slave then attempts to write the same entry, but since
myisamchk was not executed on the slave, the entry exists hence the duplication error.