diff --git a/pkg/resmap/resmap.go b/pkg/resmap/resmap.go
index 9c9e194fc..58e552137 100644
--- a/pkg/resmap/resmap.go
+++ b/pkg/resmap/resmap.go
@@ -39,8 +39,23 @@ type ResMap interface {
 	// as appended.
 	Resources() []*resource.Resource
 
-	// Append adds a Resource.
-	// Error on OrgId collision.
+	// Append adds a Resource. Error on CurId collision.
+	//
+	// A class invariant of ResMap is that all of its
+	// resources must differ in their value of
+	// CurId(), aka current Id.  The Id is the tuple
+	// of {namespace, group, version, kind, name}
+	// (see ResId).
+	//
+	// This invariant reflects the invariant of a
+	// kubernetes cluster, where if one tries to add
+	// a resource to the cluster whose Id matches
+	// that of a resource already in the cluster,
+	// only two outcomes are allowed.  Either the
+	// incoming resource is _merged_ into the existing
+	// one, or the incoming resource is rejected.
+	// One cannot end up with two resources
+	// in the cluster with the same Id.
 	Append(*resource.Resource) error
 
 	// AppendAll appends another ResMap to self,
diff --git a/pkg/resmap/resmap_test.go b/pkg/resmap/resmap_test.go
index babe8cb20..cb8742599 100644
--- a/pkg/resmap/resmap_test.go
+++ b/pkg/resmap/resmap_test.go
@@ -6,6 +6,7 @@ package resmap_test
 import (
 	"fmt"
 	"reflect"
+	"strings"
 	"testing"
 
 	"sigs.k8s.io/kustomize/v3/k8sdeps/kunstruct"
@@ -45,6 +46,24 @@ func makeCm(i int) *resource.Resource {
 		})
 }
 
+// Maintain the class invariant that no two
+// resources can have the same CurId().
+func TestAppendRejectsDuplicateResId(t *testing.T) {
+	w := New()
+	if err := w.Append(makeCm(1)); err != nil {
+		t.Fatalf("append error: %v", err)
+	}
+	err := w.Append(makeCm(1))
+	if err == nil {
+		t.Fatalf("expected append error")
+	}
+	if !strings.Contains(
+		err.Error(),
+		"may not add resource with an already registered id") {
+		t.Fatalf("unexpected error: %v", err)
+	}
+}
+
 func TestAppendRemove(t *testing.T) {
 	w1 := New()
 	doAppend(t, w1, makeCm(1))
diff --git a/plugin/builtin/namespacetransformer/NamespaceTransformer.go b/plugin/builtin/namespacetransformer/NamespaceTransformer.go
index 5bc887e5d..eb0e6732a 100644
--- a/plugin/builtin/namespacetransformer/NamespaceTransformer.go
+++ b/plugin/builtin/namespacetransformer/NamespaceTransformer.go
@@ -11,15 +11,15 @@ import (
 	"sigs.k8s.io/kustomize/v3/pkg/resmap"
 	"sigs.k8s.io/kustomize/v3/pkg/resource"
 	"sigs.k8s.io/kustomize/v3/pkg/transformers"
-	"sigs.k8s.io/kustomize/v3/pkg/types"
 	"sigs.k8s.io/kustomize/v3/pkg/transformers/config"
+	"sigs.k8s.io/kustomize/v3/pkg/types"
 	"sigs.k8s.io/yaml"
 )
 
 // Change or set the namespace of non-cluster level resources.
 type plugin struct {
 	types.ObjectMeta `json:"metadata,omitempty" yaml:"metadata,omitempty" protobuf:"bytes,1,opt,name=metadata"`
-	FieldSpecs []config.FieldSpec `json:"fieldSpecs,omitempty" yaml:"fieldSpecs,omitempty"`
+	FieldSpecs       []config.FieldSpec `json:"fieldSpecs,omitempty" yaml:"fieldSpecs,omitempty"`
 }
 
 //noinspection GoUnusedGlobalVariable
diff --git a/plugin/builtin/patchjson6902transformer/PatchJson6902Transformer.go b/plugin/builtin/patchjson6902transformer/PatchJson6902Transformer.go
index ba74db05a..e21e88d11 100644
--- a/plugin/builtin/patchjson6902transformer/PatchJson6902Transformer.go
+++ b/plugin/builtin/patchjson6902transformer/PatchJson6902Transformer.go
@@ -17,11 +17,11 @@ import (
 )
 
 type plugin struct {
-	ldr     ifc.Loader
+	ldr          ifc.Loader
 	decodedPatch jsonpatch.Patch
-	Target  types.PatchTarget `json:"target,omitempty" yaml:"target,omitempty"`
-	Path    string `json:"path,omitempty" yaml:"path,omitempty"`
-	JsonOp  string `json:"jsonOp,omitempty" yaml:"jsonOp,omitempty"`
+	Target       types.PatchTarget `json:"target,omitempty" yaml:"target,omitempty"`
+	Path         string            `json:"path,omitempty" yaml:"path,omitempty"`
+	JsonOp       string            `json:"jsonOp,omitempty" yaml:"jsonOp,omitempty"`
 }
 
 //noinspection GoUnusedGlobalVariable