I would likely open the file for *reading* and search, using a while loop and the match operator, for the 'email' record. If that exists then exit. Otherwise, seek to the beginning of the file, open a new file and write the contents of the existing file to the new file, adding the desired record at some point. How this is done would depend on the format of your file. (If it consists of lots of lines of text, then you can use the match operator to decide when to insert the desired record.) Finally, I'd rename the old file (with a .bak suffix perhaps) and the newly written file to "people.dat" or whatever.
(Opening the file in append mode will only allow you to add the new record at the end, but if that's what you want then you might do that. You won't be able to add a line in the middle of the file, though.)
(Update: Of course, do some torture tests with your code to make sure it works as desired before putting it into production.)