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which( maximum( ($a->dummy == $b) ) != 0)

which( maximum( $a->dummy == $b ) )

# The one I basically handed you. Catches duplicates.
$c = which( $a == $b->dummy ) % $a->dim(0);
...

# Using summation. Like yours, ignores duplicates.
$c = which( sumover( $a->dummy == $b ) );