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Clearly once you have run out of the temporary hit points (THPs) provided by the spell or they have been replaced by choosing to take temporary hit points from another source, creatures hitting you no longer take damage. However, RAW, the spell does not end until the 1 hour duration has expired, it just provides you with no benefits.

Armor of Agathys PHB p.215

Duration: 1 hour

A protective magical force surrounds you, manifesting as a spectral frost that covers you and your gear. You gain 5 temporary hit points for the duration. If a creature hits you with a melee attack while you have these hit points, the creature takes 5 cold damage.

The wording of the spell suggests that the spell continues to run even after the THPs are gone: "If a creature hits you with a melee attack while you have these hit points, the creature takes 5 cold damage" suggesting the spell continues but does not inflict damage, as opposed to a description such as "When the THPs are used up the spell ends".

Can you benefit from a second (or third etc.) casting of the same spell within the duration or does it need to be dispelled first?

To be clear this is not a question fishing for more THPs at one time by power gaming the spell, it is about what occurs when the THPs of the spell run out and you want to recast.

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You can have two castings of the same spell active on you but you can only benefit from one at a time.

Per the PHB (pg 205):

The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don’t combine, however. Instead, the most potent effect—such as the highest bonus—from those castings applies while their durations overlap.

Emphasis mine.

When you have already lost the temporary HP from the first casting yet you are still under the effect of the spell, the second cast takes effect and grants you the new temporary HP. In this case, you benefit from the new casting of Armor of Agathys because it has a more potent effect (5 temporary HP) than the previous casting that is still active on you (no temporary HP).

Note that you are still under the effect of the previous spell until its duration expires (this might matter for dispelling or other things), but it has no effect because you've already expended its usefulness.

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  • \$\begingroup\$ hmm... odd thought, that may wind up needing to be its own full question. Could you take a second casting of the spell while you still only have 1 THP left, giving you 2 castings (1thp, and 5thp)? You would only benifit from one at a time, but after the 5thp been used up would you then re-benefit from the 1 thp? \$\endgroup\$ – Michael Gorman Aug 28 '17 at 18:21
  • \$\begingroup\$ @MichaelGorman No, because when you replace the old THP with the new THP, the old THP are gone. \$\endgroup\$ – LegendaryDude Aug 29 '17 at 16:21
  • \$\begingroup\$ Well the text doesn't actually say that the less potent effects are gone, it says that only the most potent is applied while the durations overlap. And since THP is an odd case where potency actually changes over the duration of the spell, I am curious if this specific case has had further developer review (not that the ruling you stated wouldn't be an fair/acceptable one) \$\endgroup\$ – Michael Gorman Aug 29 '17 at 18:06
  • \$\begingroup\$ Nvm, I just saw the part of the PHB that does actually handle the THP specific case. Thus answering my own curiosity \$\endgroup\$ – Michael Gorman Aug 29 '17 at 18:10

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