In the dark [Without Sufficient Lighting], use a dirt block and hoe it / till it, making it a farmland block. Grab a seed [any kind, you only need one], and attempt to plant it. Normally, what would happen is the seed should plant and immediently fall off, but they never made it so the seed gets placed in the first place, only it popping off, making another seed, and not getting rid of your first seed, efectively duplicating the seed. You can hold the L button to spam this for as long as you'd like, as there is no limit.