The Channel Divinity is Touch of Death, which reads as follows:

When the cleric hits a creature with a melee attack, the cleric can use Channel Divinity to deal extra necrotic damage to the target. The damage equals 5 + twice his or her cleric level.

This doesn't require any action,so is there a reason why a high level death domain cleric couldn't use it multiple times on the same attack? I get that "features with the same name don't stack", but features like the paladin's divine smite specify you expend ONE spell slot, with no such specification for this feature.