The move will fail to seed the target when used on a Pokémon that is already seeded, or that is currently Grass-type.
Source
Leech Seed cannot stack.
Stack with other Hazards
Stealth Rock, Spikes, Toxic Spikes, Sticky Web, and Sharp Steel can be on the field with Leech Seed, so they can stack.
Stack with status conditions
Burn, Poison, Bad Poison, Salt Cure, Black Sludge, and Binding Damage can stack onto Leech Seeds turn-by-turn damage, although Burn/Poison/Bad Poison cannot be on a Pokémon at the same time.
Source: Experience