g/x/s/x/x prints every substituted line twice

[Douglas McIlroy]

There's nothing special about all those x's. g/x/s/y/z misbehaves, too.
Related commands produce no such surprises:
    s/x/x prints once if successful
    s/x/x/ doesn't print
    s/x/x/p prints once if successful
    g/x/s/x/x/ doesn't print
    g/x/s/x/x/p prints substituted lines once

observed in versions 1.18 and 1.20.1

Doug McIlroy