The two black wires were connected under one screw, as installed by the previous house owner. And I've never liked or trusted backstabs. Thanks for the suggestion! I've used your method with the pigtail, and Noalux (an antioxide paste), so that there are now 3 black wires in the wire nut, with only one going to the screw terminal. Having a deep box helps give space, with all the wires. I've used a wire nut made of ceramic with moulded threads for the inside copper connector (with a screw to secure the 3 twisted wires), and no internal metal coil, since they seem more secure. I hope this is ok. If not, I'll redo, using a metal coil type nut.
House was built in 1970 in Canada, but the screws of neither outlet are steel - no response to a magnet. I will check other outlets. Thanks for the idea!
I know aluminum wire has problems, but your link gives details and excellent info.
Seemingly "simple" electrical work can often be very involved, and thus can have potentially dangerous consequences, I'm now learning.
Dugie