fix: CREATE ASSOCIATION rejects duplicates for cross-module associations (#389)

The duplicate check existed only in the same-module (else) branch.
The cross-module (isCrossModule) branch went straight to CreateCrossAssociation
with no guard, silently creating a second association with the same name.

Added the same duplicate check — scanning both CrossAssociations and
Associations by name — before the cross-module create path. Added a
regression test seeding a CrossModuleAssociation in the DM and verifying
that a second CREATE returns "already exists".

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: ako

mdl/executor/cmd_associations.go

@@ -141,6 +141,18 @@ func execCreateAssociation(ctx *ExecContext, s *ast.CreateAssociationStmt) error
 	}
 
 	if isCrossModule {
+		if !s.CreateOrModify {
+			for _, ca := range dm.CrossAssociations {
+				if ca.Name == s.Name.Name {
+					return mdlerrors.NewAlreadyExists("association", s.Name.String())
+				}
+			}
+			for _, assoc := range dm.Associations {
+				if assoc.Name == s.Name.Name {
+					return mdlerrors.NewAlreadyExists("association", s.Name.String())
+				}
+			}
+		}
 		childRef := childModule + "." + s.Child.Name
 		ca := &domainmodel.CrossModuleAssociation{
 			Name:          s.Name.Name,

mdl/executor/cmd_associations_mock_test.go

@@ -166,6 +166,62 @@ func TestCreateAssociation_OrModify_UpdatesInPlace(t *testing.T) {
 	}
 }
 
+// Issue #389 — cross-module CREATE ASSOCIATION must also reject duplicates.
+func TestCreateAssociation_CrossModule_AlreadyExists_Issue389(t *testing.T) {
+	mod1 := mkModule("ModA")
+	mod2 := mkModule("ModB")
+	ent1 := mkEntity(mod1.ID, "Order")
+	ent2 := mkEntity(mod2.ID, "Product")
+
+	existingCA := &domainmodel.CrossModuleAssociation{
+		BaseElement: model.BaseElement{ID: nextID("ca")},
+		ContainerID: nextID("dm1"),
+		Name:        "Order_Product",
+		ChildRef:    "ModB.Product",
+	}
+	dm1 := &domainmodel.DomainModel{
+		BaseElement:       model.BaseElement{ID: nextID("dm1")},
+		ContainerID:       mod1.ID,
+		Entities:          []*domainmodel.Entity{ent1},
+		CrossAssociations: []*domainmodel.CrossModuleAssociation{existingCA},
+	}
+	dm2 := &domainmodel.DomainModel{
+		BaseElement: model.BaseElement{ID: nextID("dm2")},
+		ContainerID: mod2.ID,
+		Entities:    []*domainmodel.Entity{ent2},
+	}
+	h := mkHierarchy(mod1, mod2)
+	withContainer(h, dm1.ID, mod1.ID)
+	withContainer(h, dm2.ID, mod2.ID)
+	withContainer(h, ent1.ContainerID, dm1.ID)
+	withContainer(h, ent2.ContainerID, dm2.ID)
+
+	mb := &mock.MockBackend{
+		IsConnectedFunc: func() bool { return true },
+		ListModulesFunc: func() ([]*model.Module, error) {
+			return []*model.Module{mod1, mod2}, nil
+		},
+		ListDomainModelsFunc: func() ([]*domainmodel.DomainModel, error) {
+			return []*domainmodel.DomainModel{dm1, dm2}, nil
+		},
+		GetDomainModelFunc: func(id model.ID) (*domainmodel.DomainModel, error) {
+			if id == mod1.ID {
+				return dm1, nil
+			}
+			return dm2, nil
+		},
+	}
+
+	ctx, _ := newMockCtx(t, withBackend(mb), withHierarchy(h))
+	err := execCreateAssociation(ctx, &ast.CreateAssociationStmt{
+		Name:   ast.QualifiedName{Module: "ModA", Name: "Order_Product"},
+		Parent: ast.QualifiedName{Module: "ModA", Name: "Order"},
+		Child:  ast.QualifiedName{Module: "ModB", Name: "Product"},
+	})
+	assertError(t, err)
+	assertContainsStr(t, err.Error(), "already exists")
+}
+
 func TestCreateAssociation_AlreadyExists_NoOrModify(t *testing.T) {
 	mod := mkModule("MyModule")
 	ent1 := mkEntity(mod.ID, "Order")