diff --git a/api/v1alpha1/seinodedeployment_types.go b/api/v1alpha1/seinodedeployment_types.go
index 2aa97c5..19b61ff 100644
--- a/api/v1alpha1/seinodedeployment_types.go
+++ b/api/v1alpha1/seinodedeployment_types.go
@@ -9,6 +9,7 @@ import (
 //
 // +kubebuilder:validation:XValidation:rule="!has(self.genesis) || has(self.template.spec.validator)",message="genesis is meaningful only for validator-role deployments (full nodes inherit genesis from the validator ceremony's S3 artifact); remove spec.genesis or set template.spec.validator: {}"
 // +kubebuilder:validation:XValidation:rule="!has(oldSelf.genesis) || (has(self.genesis) && self.genesis == oldSelf.genesis)",message="spec.genesis is immutable once set; the ceremony's outputs (chain ID, validator gentxs, account balances) are baked into chain state and cannot be retroactively rewritten by editing the spec"
+// +kubebuilder:validation:XValidation:rule="!has(self.template.spec.validator) || !has(self.template.spec.validator.signingKey) || self.replicas == 1",message="a validator with a signingKey must have replicas: 1 — every replica mounts the same priv_validator_key.json, so >1 replica double-signs (equivocation) and tombstones/slashes the validator"
 type SeiNodeDeploymentSpec struct {
 	// Replicas is the number of SeiNode instances to create.
 	// +kubebuilder:validation:Minimum=1
diff --git a/config/crd/sei.io_seinodedeployments.yaml b/config/crd/sei.io_seinodedeployments.yaml
index 56b6d45..aec8f74 100644
--- a/config/crd/sei.io_seinodedeployments.yaml
+++ b/config/crd/sei.io_seinodedeployments.yaml
@@ -925,6 +925,11 @@ spec:
                 state and cannot be retroactively rewritten by editing the spec
               rule: '!has(oldSelf.genesis) || (has(self.genesis) && self.genesis ==
                 oldSelf.genesis)'
+            - message: 'a validator with a signingKey must have replicas: 1 — every
+                replica mounts the same priv_validator_key.json, so >1 replica double-signs
+                (equivocation) and tombstones/slashes the validator'
+              rule: '!has(self.template.spec.validator) || !has(self.template.spec.validator.signingKey)
+                || self.replicas == 1'
           status:
             description: SeiNodeDeploymentStatus defines the observed state of a SeiNodeDeployment.
             properties:
diff --git a/internal/controller/nodedeployment/envtest/validator_replicas_test.go b/internal/controller/nodedeployment/envtest/validator_replicas_test.go
new file mode 100644
index 0000000..c776d66
--- /dev/null
+++ b/internal/controller/nodedeployment/envtest/validator_replicas_test.go
@@ -0,0 +1,78 @@
+//go:build envtest
+
+package envtest_test
+
+import (
+	"testing"
+
+	. "github.com/onsi/gomega"
+
+	"sigs.k8s.io/controller-runtime/pkg/client"
+
+	seiv1alpha1 "github.com/sei-protocol/sei-k8s-controller/api/v1alpha1"
+	"github.com/sei-protocol/sei-k8s-controller/internal/controller/nodedeployment/envtest/fixtures"
+)
+
+// withSigningAndNodeKey attaches a signingKey + nodeKey to an existing
+// validator spec, referencing distinct Secrets. Distinct names satisfy the
+// signingKey/nodeKey-paired CEL rules so a test can isolate the
+// replicas==1 rule. Requires WithValidator() to have set Validator first.
+func withSigningAndNodeKey(snd *seiv1alpha1.SeiNodeDeployment) {
+	snd.Spec.Template.Spec.Validator.SigningKey = &seiv1alpha1.SigningKeySource{
+		Secret: &seiv1alpha1.SecretSigningKeySource{SecretName: "test-signing-key"},
+	}
+	snd.Spec.Template.Spec.Validator.NodeKey = &seiv1alpha1.NodeKeySource{
+		Secret: &seiv1alpha1.SecretNodeKeySource{SecretName: "test-node-key"},
+	}
+}
+
+// TestValidator_SigningKeyRequiresSingleReplica asserts the spec-level CEL
+// rule that rejects replicas > 1 when validator.signingKey is set. Every
+// replica mounts the same priv_validator_key.json, so more than one replica
+// double-signs (equivocation) and tombstones/slashes the validator.
+func TestValidator_SigningKeyRequiresSingleReplica(t *testing.T) {
+	t.Run("signingKey with replicas 1 is accepted", func(t *testing.T) {
+		g := NewWithT(t)
+		ns := makeNamespace(t)
+		snd := fixtures.NewSND(ns, "val-keys-r1", fixtures.WithValidator(), fixtures.WithReplicas(1))
+		withSigningAndNodeKey(snd)
+		g.Expect(testCli.Create(testCtx, snd)).To(Succeed(),
+			"a signing validator with replicas:1 must be accepted")
+	})
+
+	t.Run("signingKey with replicas 2 is rejected", func(t *testing.T) {
+		g := NewWithT(t)
+		ns := makeNamespace(t)
+		snd := fixtures.NewSND(ns, "val-keys-r2", fixtures.WithValidator(), fixtures.WithReplicas(2))
+		withSigningAndNodeKey(snd)
+		err := testCli.Create(testCtx, snd)
+		g.Expect(err).To(HaveOccurred(), "a signing validator with replicas:2 must be rejected")
+		g.Expect(err.Error()).To(ContainSubstring("must have replicas: 1"),
+			"rejection must carry the CEL rule message; got: %s", err.Error())
+	})
+
+	t.Run("validator without a signingKey allows replicas > 1", func(t *testing.T) {
+		g := NewWithT(t)
+		ns := makeNamespace(t)
+		// Genesis-ceremony / non-signing observer: keys are generated
+		// on-cluster, no fixed signingKey is mounted, so multiple replicas
+		// are legitimate. The rule must not catch this case.
+		snd := fixtures.NewSND(ns, "val-nokeys-r3", fixtures.WithValidator(), fixtures.WithReplicas(3))
+		g.Expect(testCli.Create(testCtx, snd)).To(Succeed(),
+			"a validator without a signingKey must allow replicas > 1")
+	})
+
+	t.Run("scaling a signing validator above 1 is rejected on update", func(t *testing.T) {
+		g := NewWithT(t)
+		ns := makeNamespace(t)
+		snd := fixtures.NewSND(ns, "val-keys-scale", fixtures.WithValidator(), fixtures.WithReplicas(1))
+		withSigningAndNodeKey(snd)
+		g.Expect(testCli.Create(testCtx, snd)).To(Succeed())
+
+		err := updateSNDWithRetry(t, client.ObjectKeyFromObject(snd), func(cur *seiv1alpha1.SeiNodeDeployment) {
+			cur.Spec.Replicas = 2
+		})
+		g.Expect(err).To(HaveOccurred(), "scaling a signing validator above 1 must be rejected")
+		g.Expect(err.Error()).To(ContainSubstring("must have replicas: 1"))
+	})
+}
diff --git a/manifests/sei.io_seinodedeployments.yaml b/manifests/sei.io_seinodedeployments.yaml
index 56b6d45..aec8f74 100644
--- a/manifests/sei.io_seinodedeployments.yaml
+++ b/manifests/sei.io_seinodedeployments.yaml
@@ -925,6 +925,11 @@ spec:
                 state and cannot be retroactively rewritten by editing the spec
               rule: '!has(oldSelf.genesis) || (has(self.genesis) && self.genesis ==
                 oldSelf.genesis)'
+            - message: 'a validator with a signingKey must have replicas: 1 — every
+                replica mounts the same priv_validator_key.json, so >1 replica double-signs
+                (equivocation) and tombstones/slashes the validator'
+              rule: '!has(self.template.spec.validator) || !has(self.template.spec.validator.signingKey)
+                || self.replicas == 1'
           status:
             description: SeiNodeDeploymentStatus defines the observed state of a SeiNodeDeployment.
             properties: