Do multiple versions of the same spell overlap or replace one another?

[drsparnum]

A L16 wizard casts a 1 hour/level duration spell (e.g., mass planar adaptation). The spell is cast while under the effects of an arcane concordance effect, so the duration is doubled to 32 total hours.

The wizard casts this spell at 7:00 AM on Monday.

On Tuesday, the wizard casts the same spell at 7:00 AM.

Does this second casting overlap with, or replace, the first casting.

Put another way, at 7:01 AM on Tuesday does the wizard have 2 mass planar adaptations up (1 with an 8 hour duration remaining, and 1 with a 32 hour duration remaining)? Or does he just have 1 mass planar adaptation up with a 32 hour duration remaining.

I'm aware that he doesn't get any stacking benefits from having 2 mass planar adaptations up simultaneously. So why am I asking? This question is really about if this wizard gets hit with greater dispel magic at 7:02 AM does the greater dispel magic have to chew through 2 castings of mass planar adaptation or just one?

[Mysterious Stranger]

By default, you would have two spells with different durations. Some spells specify that further casting extend the spells duration. Those spells add the duration to the existing duration. If that was the case, you would have 1 spell with 40 hours duration. Mass Planar Adaption does not state that, so you end up with two separate spells with different durations.

[Claxon]

Generally speaking spells overlap (with different durations) but some spells might specify different for what happens with additional castings.

[Azothath]

this is an old topic. Basically you have two cases; where the spell description discusses it and where it does not (the basic or generic case). The Game is well known for quirky spell interactions.

Where the description discusses it may or may not allow a new save (options like; extend, reset duration by same caster, cancel|counter or dismiss, etc). Again read the description.

With the basic case you have two spells with an overlapping area of effects and durations, different DCs and saves. GMs should ask if the original caster dismisses the first spell (look for the "D" in the spell description block). Usually a caster produces consistent effects so the spell's numbers stay the same.

Planar Adaptation:T(4-5). The spell description does not discuss extending the spell or recasting. Thus you are in the basic case. This spell is also known for its poor wording implying that it must be cast on a (hazardous) plane rather than precast for a targeted destination.