sehgalanuj

I'm not whom you asked the question to, but I have recent experience regarding double booking on the same flight for the same passenger. The FB# was even added to the booking, seats selected, everything. Neither of the bookings was cancelled and stayed normal until I finally decided to cancel one of them. I even spoke to staff at the PSL to actively cancel the original booking (made with points), and they themselves told me that it is not advisable to do so since converting to a one-way reward was far too expensive at that point. Instead they said it was best to keep the revenue return, and reward return, both active. When asked about pro-active cancellation, they said that they do have automatic systems to check for this but those usually only kick in during the first few weeks. If in the first weeks a ticket doesn't get cancelled, then it is unlikely that it will.

So officially, it shouldn't be done. There are systems to detect it but apparently not aggressive. It is highly likely that you will be ok, but it is also possible that one of the bookings ends up being cancelled.

EDIT: Earlier in the year I was on a DEL-CDG-HAM (revenue) flight and needed to change it to DEL-CDG-NUE. There was only one seat available on the CDG-NUE flight, and within what I was ok paying miles for as well. Online change wasn't working, so I bought the ticket in miles to at least hold the seat. Then called the PSL, and was advised to just leave it as is, with DEL-CDG-HAM (revenue) and CDG-NUE (redemption) - while in CDG taking the CDG-NUE flight instead of CDG-HAM. The PSL agent didn't want to cancel my CDG-NUE redemption booking because he was afraid that with just one seat, if he cancelled my flight it might have taken time for inventory to become available again and someone else might have snagged it while we waited. So I flew this way, and all was ok. That said, this is not something I would abuse since it really is not permitted on most fares and I did it in a highly exceptional circumstance.