Does a replacement effect that doubles counters placed on a creature apply to counters placed by other replacement effects?

Replacement effects are governed by CR 614. When multiple replacement effects could modify the same event, they are applied one at a time in a specific order chosen by the affected player (or the controller of the permanent being affected), per CR 616.1. Crucially, each replacement effect can only apply once to a given event (CR 614.5).

A 'doubling' replacement effect — such as that from Doubling Season — modifies how many counters are placed. If another replacement effect first changes what counters are placed (for example, converting +1/+1 counters to -1/-1 counters, or adding extra counters), the doubling effect can still apply to the resulting modified event, provided it hasn't already been applied to that same event.

The key rule is CR 614.5: a replacement effect can only modify a given event once. So if Doubling Season already doubled the counters being placed, it won't double them again due to a subsequent replacement effect on the same placement event. But if two separate replacement effects each only apply once, they can both contribute to the final result.

Concrete example: A spell says 'place 1 +1/+1 counter on target creature.' Replacement Effect A (e.g., a proliferate-like effect) changes this to 2 counters. Replacement Effect B (Doubling Season) then doubles that to 4 counters. Both effects applied legally because each applied exactly once to the event, per CR 616.1 and 614.5.

If instead the same Doubling Season tried to apply twice to the same event, it would be blocked by CR 614.5. So the answer depends on whether the doubling effect has already been applied to that specific event — if not, it can apply to counters added by another replacement effect.