A utility to append content into a file, if this content isn't already in the file (even in part)

Is there a *Nix **utility** by which I could append content to a file (say, append the content of a template file into the end of a second file), only if the content isn't already available in the second file, partially or fully? One could use this grep solution but I find the code there a bit complicated at the moment and would prefer a dedicated utility if there is one.