Alt D Bug? Linked Duplicate Bug?

Solved. Doing a 3D view manipulation showed it was mirrored, just in a different plane.

I’m doing the manual demonstration at:

Just to the bottom of the 4th screen shot.

Basically I created a plane, removed some vertices to get a curved line.

Then Alt-D Changed Transform properties to have a mirror.
The extruded some vertices and ended up at the top, but the linked
line did not do exactly like the other side.

Screenshot attached.