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LDC-E1338 — Instance name already declared

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# LDC-E1338 — Instance name already declared

## What it means

An instance introduces its reference name into the package namespace:
generated identifiers derive from it, and references resolve through
it. A name that a declaration already owns cannot be taken again — the
two owners would answer under one name. The compiler stops at the
instance.

The repair is a rename: give the instance a name nothing else
declares. Generated identifiers then derive without collision.

## Example

```law
language "law.core" version "0.2";
package demo.diagnostics version "0.1.0";
namespace "urn:law:demo:diagnostics";

entity Person;
entity Choice;
entity SpecialChoice : Choice;
entity Other;
const Selected: SpecialChoice = entity_ref("urn:choice:selected");
const Wrong: Other = entity_ref("urn:choice:wrong");
relation eligible(p: Person, choice: Choice) kind empirical;
relation applicable(p: Person, choice: Choice) kind institutional;
expansion fixed {
    params { subject: binder; choice: value; ground: relation(subject, choice); result: relation(subject, choice); }
    emit rule self/result defeasible {
        for subject;
        when ground(subject, choice);
        then result(subject, choice);
        scope from self; effective from self; labels from self; source from self;
    }
}
expand fixed Selected {
    label en official "Fixed option";
    bind subject = person: Person;
    choice = Selected;
    ground = eligible;
    result = applicable;
    effective [@2026-01-01, @2027-01-01);
}
```

## Fix

```law
language "law.core" version "0.2";
package demo.diagnostics version "0.1.0";
namespace "urn:law:demo:diagnostics";

entity Person;
entity Choice;
entity SpecialChoice : Choice;
entity Other;
const Selected: SpecialChoice = entity_ref("urn:choice:selected");
const Wrong: Other = entity_ref("urn:choice:wrong");
relation eligible(p: Person, choice: Choice) kind empirical;
relation applicable(p: Person, choice: Choice) kind institutional;
expansion fixed {
    params { subject: binder; choice: value; ground: relation(subject, choice); result: relation(subject, choice); }
    emit rule self/result defeasible {
        for subject;
        when ground(subject, choice);
        then result(subject, choice);
        scope from self; effective from self; labels from self; source from self;
    }
}
expand fixed Result {
    label en official "Fixed option";
    bind subject = person: Person;
    choice = Selected;
    ground = eligible;
    result = applicable;
    effective [@2026-01-01, @2027-01-01);
}
```

## Engine message

The engine reports this in its own wording:

```text
example.law:22:14: error LDC-E1338: expand Selected: instance name is already declared in the package (§279.1.4)
```

## Related

- [LDC-E1201](/diagnostics/LDC-E1201/) — the companion collision for ordinary declarations.
- [LDC-E1337](/diagnostics/LDC-E1337/) — the instance must honor the definition contract.