Why Logstash reload the same data many times?

Hello,

Every update I delete the files and I create new ones every day at 10pm.

but I have an others files (in the input section I have many files ) witche updated every 30 seconds, in this case It insering the new data to the same file, and I delete the file once per week.

If I delete sincedb_path => "/dev/null" the process works, no problem. Can you please explain to me what does it mean this command ? and if there is an other command to force logstash to load data just one time ?

thanks.