Budoka Gardener flips even if you put a second copy of a legendary land onto the battlefield. You’ll have ten lands on the battlefield while the ability is resolving, so the Gardener flips. Then you will choose one of the legendary lands to keep and put the other into its owner’s graveyard as a state-based action.

Budoka Gardener flips if you control ten or more lands when its ability resolves, even if you don’t use its ability to put a land onto the battlefield.