Does undying and persist interact when a creature has both abilities?

When a creature with both undying and persist would die, both abilities create replacement effects that would modify that event. Since two replacement effects apply to the same event, the affected object's controller chooses which one to apply first, per CR rule 616.1.

If the creature has no +1/+1 or -1/-1 counters, both replacement effects are eligible. The controller chooses one — say, undying. The creature returns with a +1/+1 counter. Now persist's condition ("had no -1/-1 counters") is no longer relevant to this instance, but the key nuance is that once one replacement effect applies, the other replacement effect may or may not still apply depending on whether its condition is still met.

Critically, +1/+1 counters and -1/-1 counters annihilate each other (CR 121.3). If you apply undying first (creature returns with a +1/+1 counter), persist cannot trigger again immediately. But if the creature dies again with no counters, both are again applicable. This is the core of the classic 'persist-undying' engine: the counters cancel out, letting the creature loop between both abilities repeatedly.

Concrete example: You control a Woodfall Primus (persist) enchanted with an aura granting it undying. It dies with no counters. You choose to apply persist first — it returns with a -1/-1 counter. Then a +1/+1 counter would be placed by undying, but persist already applied, so undying's replacement was not applied. The -1/-1 and +1/+1 would cancel (CR 121.3) if both somehow landed. In practice, only one replacement effect applies per death event, but the counter annihilation makes it loop.

The interaction is fully defined by CR 614.1 (replacement effects), CR 616.1 (player chooses among applicable replacements), and CR 121.3 (counter annihilation), making this a HIGH-confidence ruling.