Duplicate lines in special conditions

See the two vids below pls, how can I duplicate lines in a plane that I created?! I turned off the body in the 2nd attemp(2nd vid), n double tapped on the plane, but no difference observed.

Sketches by default will move on the plane that they were created on. You can project that line to your sketch plane, that will add that line to the same plane.

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Will try. Thanks.