Some spells, like true polymorph, has language that says (emphasis mine)
If you concentrate on this spell for the full duration, the spell lasts until it is dispelled.
This seems to suggest that the duration is over and the spell simply lasts until dispelled. If you are looking at the rules around Combining Spell Effects, does "lasts until dispelled" count as a duration? Or is it outside of that mechanic and the spell actually has no ongoing 'duration'?
Player's Handbook, p.203, "Duration":
A spell's duration is the length of time the spell persists. A duration can be expressed in rounds, minutes, hours, or even years. Some spells specify that their effects last until the spells are dispelled or destroyed.
I think there is a misunderstanding here (also inhowfar the question is asked). That's not your fault though, the wording of the spell description is pretty bad.
It says that the duration is for as long as you concentrate (up to 1 hour). It also says that if you concentrate for the full duration, then the spell lasts until it is dispelled (which arguably means the "duration" extends, so you need to keep concentrating or else it will not last until dispelled!).
Now, what the text meant to say, obviously, is that it lasts for as long as you concentrate, and if that is less than an hour, the effect ceases to exist.
But if you concentrate for the full hour, then the effect becomes quasi-permanent (without further concentrating). Until dispelled.
Otherwise, it doesn't make any sense at all.
Is "until dispelled" a duration now, or is it not? I beg to differ with Quadratic Wizard's point of view. Although the quoted paragraph does start with "A spell's duration is...", it doesn't say that "until dispelled" counts as duration at all.
It speaks of rounds, minutes, hours, or years as duration. And then, it states that some spells last until dispelled (i.e. have no such duration).
No, lasts until dispelled is not a duration
The duration of a spell is explicitly given in the spell text:
Duration: Concentration, Up to 1 hour
The effects of a spell can last longer, as stated in the rules - in this case potentially until dispelled, but the spell itself is no longer active, only the effects of it. For example, the hit points cured by cure wounds persist until destroyed, but that doesn’t mean that the cure wounds spell’s “instantaneous” duration is extended until that happens. The effects linger, but the spell that created the effect has ended.
Spells only do what they say they do. In this case, the spell says its duration is up to an hour, and its effects can last until dispelled. The spell text does not say its duration is extended indefinitely if you concentrate for an hour, so it is not.
For the purposes of combining spell effects
The second casting of True Polymorph will suppress the first casting. If that spell is dropped before an hour passes, the target will revert to the form it took from the first True Polymorph. If the second casting is concentrated on for a full hour, the from it took from the second casting will persist as well, suppressing the effect of the first until it is dispelled.
