inser_line_after not working correctly?

Hi.
The Readme states, that "insert_line_after" would "not append the line if it is in the file already". This doesn't seem to work for me atm. For every test run on the same server via test kitchen the line gets added again to the config which is subsequently killing the configuration and therefore the chef run/provisioning of the server.
Problem occurs with chef_zero, Chef Version 12.19.36 with Kitchen 1.15.0 and Vagrant Driver