Repeated record every time new events happens

I have a very basic pipeline.config:
input {
file {
path => ["/var/log/mylog.log"]
start_position => "beginning"
}
}

filter {

}
output {
elasticsearch {
hosts => ["localhost:9200"]
index => "test"
}
}
lets assume in my /var/log/mylog.log there are below records
line1
line2
when i run logstash -f /etc/logstash/pipeline.config everything is okey but if i added line3 to /var/log/mylog.log all lines repeated to my index and i got repeated data..what is wrong?

Did you restart logstash? It may be the same problem described in this thread, in which case there is no fix at this time.

Well i stoped the logstash service in order to make sure no othe process is running.. And run this pipeline by logstash binary.