Alex is playing against Neil. This is the precombat main phase of Alex. He has a Doubling Season under his control. He casts Makeshift Mannequin, targeting his Epochrasit in his graveyard. All players pass piority. What will happen?

Doubling Season
Enchantment, 4G (5)
If an effect would put one or more tokens onto the battlefield under your control, it puts twice that many of those tokens onto the battlefield instead.

If an effect would place one or more counters on a permanent you control, it places twice that many of those counters on that permanent instead.

Makeshift Mannequin
Instant, 3B (4)
Return target creature card from your graveyard to the battlefield with a mannequin counter on it. For as long as that creature has a mannequin counter on it, it has “When this creature becomes the target of a spell or ability, sacrifice it.”

Epochrasite
Artifact Creature — Construct 1/1, 2 (2)
Epochrasite enters the battlefield with three +1/+1 counters on it if you didn't cast it from your hand.
When Epochrasite dies, exile it with three time counters on it and it gains suspend. (At the beginning of your upkeep, remove a time counter. When the last is removed, cast this card without paying its mana cost. It has haste.)

Local L2s don't seem to agree with each other…Some think that Epochrasite enters the battefield with 2 mannequin counters and 6 +1/+1 counters, while some think that because Doubling Season only takes effect once per event, it enters the battlefield with 1 mann counter and 6 +1/+1 counters or with 2 mann counter and 3 +1/+1 counters, depending on Alex. I think Alex can decide how it enters the battlefield, with a 2/6, 1/6 or 2/3 distribution on mann counter/ +1/+1 counter, the reason being that he may let the replacement effect of mann counter takes place first, and then let the effect of Doubling Season take place(After that the +1/+1 effect) to get 2/3, while he can also let the effect of +1/+1 counters take place, then the Doubling effect to get 2/6, and vice versa.

Any help is appreciated.

Under these conditions, Epochrasite will always enter the battlefield with two mannequin counters, and it will enter with either three or six +1/+1 counters, whichever Alex wants. (So probably six.)

Makeshift Mannequin attempts to return Epochrasite to the battlefield with a mannequin counter on it. That means there are two replacement effects to be applied here: one from Doubling Season and one from Epochrasite itself.

If Doubling Season is applied first, it doubles the one mannequin counter to two. Then you apply Epochrasite's replacement, adding three +1/+1 counters to the mix. Doubling Season has already applied to Epochrasite entering the battlefield, and will not apply again, even though there's suddenly more counters around than there used to be. So Epochrasite enters with only three +1/+1 counters.

If, on the other hand, Epochrasite's replacement is applied first, you have Epochrasite about to enter with one mannequin counter and three +1/+1 counters, which is then replaced by Doubling Season, resulting in two mannequin counters and six +1/+1 counters.