In Mordenkainen's Tome of Foes (p. 129), the Deathlock Mastermind has the following attack:
Grave Bolts. Ranged Spell Attack: +6 to hit, range 120 ft., one or two targets. Hit: 18 (4d8) necrotic damage.
Can the Mastermind attack one creature with two bolts in a single round? Or is this specifically different from multiattack so that the Mastermind gets the benefit of multiple attacks if multiple targets are present?
The Deathlock Mastermind was updated in Monsters of the Multiverse, and Modenkainen's Tome of Foes is now obsolete. With this update, instead of Grave Bolts saying "one or two targets", it is now "Grave Bolt", having "one target", and is included in the Multiattack action:
Multiattack. The deathlock makes two Deathly Claw or Grave Bolt attacks.
Grave Bolt. Ranged Spell Attack: +6 to hit, range 120 ft., one target. Hit: 13 (3d8) necrotic damage.
So when the Deathlock uses Multiattack, it can just pick the same target for each attack with Grave Bolt.
Despite the fact that this seems to modeled after the Warlock's Eldritch Blast (which can attack the same target multiple times), there is no special rule that allows the monster to attack a target twice with the same attack.
For reference, Eldritch Blast states the following:
The spell creates more than one beam when you reach higher levels. [...] You can direct the beams at the same target or at different ones. Make a separate Attack roll for each beam.
You'll notice that this clarifying text is missing from the text for the Deathlock Mastermind's Grave Bolts.
