Gemini Monsters

Gemini Monsters are written with the official word "Gemini", but the local MDPro4/YGOPro constants and helpers still use the older "Dual" names. That is why scripts check TYPE_DUAL, register EFFECT_DUAL_SUMMONABLE, and test SUMMON_TYPE_DUAL.

The important gameplay split is simple:

State	Script meaning
Not in Dual State	The monster is treated as a Normal Monster while face-up in the Monster Zone or in the GY.
In Dual State	The monster has gained its Gemini effects.

Use official wording when explaining cards, but use the local constants when writing scripts. There is no local TYPE_GEMINI constant.

The Standard Setup

Most Gemini Monsters begin with aux.EnableDualAttribute:

function s.initial_effect(c)
    aux.EnableDualAttribute(c)

    local e1=Effect.CreateEffect(c)
    e1:SetType(EFFECT_TYPE_IGNITION)
    e1:SetRange(LOCATION_MZONE)
    e1:SetCondition(aux.IsDualState)
    e1:SetTarget(s.target)
    e1:SetOperation(s.operation)
    c:RegisterEffect(e1)
end

The helper registers the standard local Gemini package:

Helper piece	What it does
EFFECT_DUAL_SUMMONABLE	Allows the core to offer the face-up monster as a Normal Summon target again.
EFFECT_ADD_TYPE with TYPE_NORMAL	Treats the monster as Normal while it is not in Dual State.
EFFECT_REMOVE_TYPE with TYPE_EFFECT	Removes Effect Monster type while it is not in Dual State.

Local procedure.lua applies those type changes in LOCATION_MZONE+LOCATION_GRAVE, and only while the monster is face-up and not already in Dual State.

Gemini Soldier (c68366996.lua) is a clean local example: it calls aux.EnableDualAttribute(c) first, then gives the monster battle protection and a Deck Special Summon trigger only while aux.IsDualState is true.

Reading Dual State

There are two common checks:

Check	Use it when
aux.IsDualState	You need an Effect condition for the handler. It also checks that the handler is not disabled.
Card.IsDualState	You are filtering or inspecting a Card object directly.

Effects that only work after the monster has gained its Gemini effects usually use the helper directly:

local e1=Effect.CreateEffect(c)
e1:SetType(EFFECT_TYPE_SINGLE)
e1:SetProperty(EFFECT_FLAG_SINGLE_RANGE)
e1:SetRange(LOCATION_MZONE)
e1:SetCode(EFFECT_INDESTRUCTABLE_COUNT)
e1:SetCondition(aux.IsDualState)
e1:SetCountLimit(1)
e1:SetValue(s.indval)
c:RegisterEffect(e1)

For filters, check the Card object:

function s.gemini_filter(c)
    return c:IsFaceup() and c:IsType(TYPE_DUAL) and not c:IsDualState()
end

Gemini Booster (c18096222.lua) uses that shape before calling Card.EnableDualState on the selected monster.

Making A Gemini Gain Its Effects

Normal Summoning a face-up Gemini Monster again is handled by the core when the card has EFFECT_DUAL_SUMMONABLE and is not already affected by EFFECT_DUAL_STATUS. The resulting Summon type is SUMMON_TYPE_DUAL.

Some card effects say that a Gemini Monster is treated as having been Normal Summoned again. Those scripts do not perform a Normal Summon. They apply the state directly:

function s.operation(e,tp,eg,ep,ev,re,r,rp)
    local tc=Duel.GetFirstTarget()
    if tc and tc:IsRelateToEffect(e) then
        tc:EnableDualState()
    end
end

Gemini Ablation (c80758812.lua) shows the safer version for a monster Special Summoned during resolution:

local tc=g:GetFirst()
if tc and Duel.SpecialSummonStep(tc,0,tp,tp,false,false,POS_FACEUP) then
    tc:EnableDualState()
end
Duel.SpecialSummonComplete()

Card.EnableDualState registers EFFECT_DUAL_STATUS with a client hint. It does not Summon the monster, spend a Normal Summon, or make a Chain Link by itself. Use it only when the card text really grants the gained-effect state.

Checking For A Gemini Summon

If a card cares that a monster was Normal Summoned again as a Gemini Monster, check the Summon type:

function s.sumcon(e,tp,eg,ep,ev,re,r,rp)
    return e:GetHandler():IsSummonType(SUMMON_TYPE_DUAL)
end

Use Card.IsSummonType for that question. Do not infer Gemini Summon history from IsDualState() alone. A card effect can enable Dual State without performing a Summon, and a monster can lose or reset effects later.

Searching Gemini Cards And Support

Use TYPE_DUAL for actual Gemini Monsters:

function s.spfilter(c,e,tp)
    return c:IsType(TYPE_DUAL)
        and c:IsCanBeSpecialSummoned(e,0,tp,false,false)
end

Support Spells and Traps that mention Gemini Monsters can mark their text type for local search helpers:

s.has_text_type=TYPE_DUAL

function s.thfilter(c)
    return c:IsType(TYPE_SPELL+TYPE_TRAP)
        and aux.IsTypeInText(c,TYPE_DUAL)
        and c:IsAbleToHand()
end

Geminize Lord Golknight (c72305034.lua) uses aux.IsTypeInText this way when it searches Gemini-related Spell/Trap Cards.

Common Mistakes

Mistake	Better pattern
Using TYPE_GEMINI.	Use local TYPE_DUAL.
Forgetting aux.EnableDualAttribute(c).	Register it before the monster's Gemini-only effects.
Checking only c:IsDualState() in an Effect condition.	Use aux.IsDualState so disabled handlers do not keep applying Gemini-only effects.
Assuming Special Summon makes a Gemini Monster gain its effects.	It stays not in Dual State unless the script calls EnableDualState() or it is Normal Summoned again.
Treating EnableDualState() as a Summon.	It only applies EFFECT_DUAL_STATUS; it does not spend or create a Normal Summon.

The mental model is: TYPE_DUAL finds the subtype, aux.EnableDualAttribute gives the card the local Gemini rules, and Dual State decides whether its Gemini effects are live.