Death Knell is a spell that among other things grant a temporary +1 increase to caster level.
Are they right or wrong?
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This has been answered in a FAQ:
Temporary Hit Points: Do temporary hit point from the same source stack?
No. Generally, effects do not stack if they are from the same source (Core Rulebook page 208, Combining Magical Effects). Although temporary hit points are not a "bonus," the principle still applies.
This prevents a creature with energy drain (which grants the creature 5 temporary hit points when used) from draining an entire village of 100 people in order to gain 500 temporary hit points before the PCs arrive to fight it.
Temporary hit points from different sources (such as an aid spell, a use of energy drain, and a vampiric touch spell) still stack with each other.
—Pathfinder Design Team, September 2013
Although the FAQ is about temporary hit points, the principle is that bonus from the same source should not stack, as described on the Core Rulebook, under Combining Magical Effects.
Spells that provide bonuses or penalties on attack rolls, damage rolls, saving throws, and other attributes usually do not stack with themselves. More generally, two bonuses of the same type don't stack even if they come from different spells (or from effects other than spells; see Bonus Types, above).
Even when an effect is not labelled as a "bonus", if the effect increases it, it is gaining a bonus. Just like the spell grants you bonus temporary hit points (normally you have 0).
Temporary Hit Points, as this answer notes, have been FAQ-errataed to be treated as if they were a bonus for the purpose of self-stacking, and from that the FAQ draws the conclusion that they no longer self-stack. Thus, the rule:
Bonuses without a type always stack, unless they are from the same source.
is interpreted as meaning to imply that bonuses and temporary hp from spells never stack with themselves unless the spell specifically says so, even though in actuality those rules merely state that untyped bonuses behave thusly (and the rules for typed bonuses mean that most other types of bonuses work the same way as well, but notably not dodge bonuses)
Death Knell's increase to caster level, notably, is not written as a bonus. Anything that's not a bonus is not affected by the rule that untyped bonuses don't stack with themselves. The reasoning in the FAQ may lead us to believe that designers intend no beneficial effects of spells to self-stack, specifically because that situation can lead to what they consider problematic village-slaughtering outcomes, but just as the rules clearly do not prevent temporary hp from self-stacking without that FAQ, they also do not prevent not-bonus increases from self-stacking.