Frequently when an opponent is taking a corner kick, an opponent will have a player stand right beside the keeper to try to beat the keeper to balls (ok let’s be honest here, the real reason frequently is to obstruct the keeper as well).
The dilemma for the keeper is whether to have a player mark up this opponent or to leave this player unmarked and for the keeper to take full responsibility for this player.
It’s my recommendation to leave the player unmarked because bringing another player into this concentrated area just brings one more obstacle for the keeper to have to work around to get to the ball. While this puts some additional pressure on the keeper, it also allows the keeper to have a better understanding of what they do and don’t have to do so there is no confusion.
It’s important that all players on the team know whether an opponent right on the keeper during corner kicks should be marked or not. There isn’t really a right or wrong answer but it’s important that a decision be made before the situation arises rather than having confusion during the game.
Too many keepers get into pushing battles with the opponent who is standing beside them during corner kicks. They become more concerned with winning the "space battle" than in trying to keep the ball out of the net. The real key is to concentrate on the ball and don’t let the opponent become a major distraction.