After the repository is added, the ${::apt::sources_list_d}/${sources_list_d_filename...

After the repository is added, the ${::apt::sources_list_d}/${sources_list_d_filename} file is created as an empty file.
The unless condition of Exec["add-apt-repository-${name}"] calls test -s, which returns 1 if the file is empty. Because
the file is empty, the unless condition is never true and the repository is added on every execution. This commit
replaces the -s test condition with -f, which is true if the file exists or false otherwise.