: Select any element and click the "Repeat Grid" button in the right-hand property inspector to instantly replicate it vertically or horizontally.
: Hover your mouse cursor over the top edge of your active artboard until it transforms into a two-way vertical arrow. Click and drag it down. Adobe XD v41.0.12
: Hover your mouse cursor over the left edge of your active artboard until it transforms into a two-way horizontal arrow. Click and drag it to your desired position on the artboard. : Select any element and click the "Repeat
Keep in mind that Adobe has shifted focus away from this software. Adobe XD v41.0.12