I've done it three times, although when I went to the embassy in Doha they told me that they wouldn't do it again (actually he said that they wouldn't do it more than twice). (This was after I made an appointment on line, filled out the form, drove there, went through all three separate security checkpoints, and waited some more. Of course it doesn't say on the form that there is a limit. )

There have been many posts on FlyerTalk about this subject, and members have reported all sorts of different experiences -- some say that three times is the limit, others have reported success with more than this.