[RPG] Do multiple sources of temporary hit points combine, or do you only get one set

From the Player's Basic Rules, page 76:

If you have temporary hit points and receive more of them, you decide whether to keep the ones you have or to gain the new ones.

So in this situation you can choose to have 5 temporary hit points or 10 - it's up to you.

Armor of Agathys specifies that its effect of damaging creatures that attack you happens 'while you have these hit points'. If you replace the hit points it gave you with different hit points, you don't 'have these hit points' any more, you have some other set of hit points, so you wouldn't get the damage effect.

Cases like this are presumably why you get to choose between temporary hit points instead of automatically getting the larger amount.

This one is a little bit more tricky to answer than your previous question about Temp HP from one effect canceling out any others that rely on Temp HP from another effect. (Found here: Dark One's Blessing and Armor of Agathys Synergy? )

So then, you have to think in order of events. Combat happens very quickly (an entire round happens in the span of 6 seconds) and things don't really happen at once when it comes down to the rules. The creature doesn't hit you, take damage, your Armor of Agathys goes away but you gain more temp HP because the Armor of Agathys damage killed it and you reaped it's energy with Dark Ones Blessing.

It more happens like this: A creature attacks you, your Armor of Agathys is holding at 1hp , barely there anymore, 1 damage is eaten up by the barrier and the remaining 4 cleave into your shoulder (Since he broke the barrier with damage remaining we assume teh attack follows through, if only a little bit.) and the creature takes cold damage as a result of the Armor of Agathys doing it's job. It falls over, frozen and dead, and the Dark one blesses you with extra HP. After the creature dies, and after it's attack and damage is resolved.

I hope that answers your question?