Procedure
1)Create /tmp/test.log
touch /tmp/test.log

2)To start Zabbix server and Zabbix agent

3)item registration of /tmp/test.log
log[/tmp/test.log]

4)Add to /tmp/test.log a message
echo "test messages 1"> /tmp/test.log
-> Monitoring of message1 is ok

5)stop Zabbix server

6)Add to /tmp/test.log a message
echo "test messages 3"> /tmp/test.log
echo "test messages 4"> /tmp/test.log

7)To start Zabbix server
-> Monitoring of message3 is ok
-> Monitoring of message4 is ok

8)stop Zabbix agent

9)Add to /tmp/test.log a message
echo "test messages 5"> /tmp/test.log

10)start Zabbix agent
-> Monitoring of message3 is duplicate
-> Monitoring of message4 is duplicate
-> Monitoring of message5 is ok