Replace by fee only allows replacement of a TX to the very same output address as the lower fee original TX, so there is no double-spend risk.
The only kind of "double spend" is sending to the SAME output twice, to the recipient will still get the BTC regardless.
Look at the linked "doublespend.py" and you can see there is only ONE address you are sending to, not two.
There is a lot of FUD about this.

If you see the original TX to one of your addresses, there is no way for the sender to replace that with an alternate higher-fee TX to a different address. The only purpose is to bump the fee, leaving everything else the same, as a way to increase priority of a TX.

FUD aside, it would be handy if the Electrum UI gave users the opportunity to try sending with a low fee, and if they get impatient they can resubmit with higher fee to speed up the TX.