diff --git a/apis/zalando.org/v1/types.go b/apis/zalando.org/v1/types.go
index 32ec2e7..fa1aeb8 100644
--- a/apis/zalando.org/v1/types.go
+++ b/apis/zalando.org/v1/types.go
@@ -43,6 +43,7 @@ type RouteGroupList struct {
 type RouteGroupSpec struct {
 	// List of hostnames for the RouteGroup
 	// +kubebuilder:validation:MinItems=1
+	// +kubebuilder:validation:XValidation:rule="self.all(i, size(self.filter(j, j==i)) == 1)", message="hosts can not contain duplicates"
 	Hosts []string `json:"hosts,omitempty"`
 	// List of backends that can be referenced in the routes
 	Backends []RouteGroupBackend `json:"backends"`
diff --git a/zalando.org_routegroups.yaml b/zalando.org_routegroups.yaml
index 04236eb..1b2b264 100644
--- a/zalando.org_routegroups.yaml
+++ b/zalando.org_routegroups.yaml
@@ -136,6 +136,9 @@ spec:
                   type: string
                 minItems: 1
                 type: array
+                x-kubernetes-validations:
+                - message: hosts can not contain duplicates
+                  rule: self.all(i, size(self.filter(j, j==i)) == 1)
               routes:
                 description: Routes describe how a matching HTTP request is handled
                   and where it is forwarded to