Add validation for case insensitive header name repetition in filters

Author: ciarams87

internal/mode/static/state/graph/httproute.go

@@ -836,6 +836,20 @@ func validateFilterHeaderModifierFields(
 ) field.ErrorList {
 	var allErrs field.ErrorList
 
+	// Ensure that the header names are case-insensitive unique
+	allErrs = append(allErrs, validateRequestHeadersCaseInsensitiveUnique(
+		headerModifier.Add,
+		headerModifierPath.Child("add"))...,
+	)
+	allErrs = append(allErrs, validateRequestHeadersCaseInsensitiveUnique(
+		headerModifier.Set,
+		headerModifierPath.Child("set"))...,
+	)
+	allErrs = append(allErrs, validateRequestHeaderStringCaseInsensitiveUnique(
+		headerModifier.Remove,
+		headerModifierPath.Child("remove"))...,
+	)
+
 	for _, h := range headerModifier.Add {
 		if err := validator.ValidateRequestHeaderName(string(h.Name)); err != nil {
 			valErr := field.Invalid(headerModifierPath.Child("add"), h, err.Error())
@@ -865,3 +879,40 @@ func validateFilterHeaderModifierFields(
 
 	return allErrs
 }
+
+func validateRequestHeadersCaseInsensitiveUnique(
+	headers []v1.HTTPHeader,
+	path *field.Path,
+) field.ErrorList {
+	var allErrs field.ErrorList
+
+	seen := make(map[string]struct{})
+
+	for _, h := range headers {
+		name := strings.ToLower(string(h.Name))
+		if _, exists := seen[name]; exists {
+			valErr := field.Invalid(path, h, "header name is not unique")
+			allErrs = append(allErrs, valErr)
+		}
+		seen[name] = struct{}{}
+	}
+
+	return allErrs
+}
+
+func validateRequestHeaderStringCaseInsensitiveUnique(headers []string, path *field.Path) field.ErrorList {
+	var allErrs field.ErrorList
+
+	seen := make(map[string]struct{})
+
+	for _, h := range headers {
+		name := strings.ToLower(h)
+		if _, exists := seen[name]; exists {
+			valErr := field.Invalid(path, h, "header name is not unique")
+			allErrs = append(allErrs, valErr)
+		}
+		seen[name] = struct{}{}
+	}
+
+	return allErrs
+}

internal/mode/static/state/graph/httproute_test.go

@@ -2388,6 +2388,25 @@ func TestValidateFilterRequestHeaderModifier(t *testing.T) {
 			expectErrCount: 7,
 			name:           "request header modifier filter all fields invalid",
 		},
+		{
+			validator: createAllValidValidator(),
+			filter: gatewayv1.HTTPRouteFilter{
+				Type: gatewayv1.HTTPRouteFilterRequestHeaderModifier,
+				RequestHeaderModifier: &gatewayv1.HTTPHeaderFilter{
+					Set: []gatewayv1.HTTPHeader{
+						{Name: "MyBespokeHeader", Value: "my-value"},
+						{Name: "mYbespokeHEader", Value: "duplicate"},
+					},
+					Add: []gatewayv1.HTTPHeader{
+						{Name: "Accept-Encoding", Value: "gzip"},
+						{Name: "accept-encodING", Value: "gzip"},
+					},
+					Remove: []string{"Cache-Control", "cache-control"},
+				},
+			},
+			expectErrCount: 3,
+			name:           "request header modifier filter not unique names",
+		},
 	}
 
 	filterPath := field.NewPath("test")