You can set the following two parameters in your file input:
sincedb_path => "/dev/null"
start_position => "beginning"
The first sets the sincedb_path file to /dev/null, which means that logstash won't be able to save the place it was up to in the file between runs. The second sets the start_postion to the beginning of the file, so it will always read from the start of the file.