diff --git a/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/ProtobufSchemaCodegen.java b/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/ProtobufSchemaCodegen.java
index b3fb96b7f11..314b9a2f359 100644
--- a/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/ProtobufSchemaCodegen.java
+++ b/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/ProtobufSchemaCodegen.java
@@ -272,7 +272,7 @@ public class ProtobufSchemaCodegen extends DefaultCodegen implements CodegenConf
         if (additionalProperties.containsKey(CUSTOM_OPTIONS_MODEL)) {
             this.setCustomOptionsModel((String) additionalProperties.get(CUSTOM_OPTIONS_MODEL));
         }
-          
+
         if (additionalProperties.containsKey(this.SUPPORT_MULTIPLE_RESPONSES)) {
             this.supportMultipleResponses = convertPropertyToBooleanAndWriteBack(SUPPORT_MULTIPLE_RESPONSES);
         } else {
@@ -546,22 +546,32 @@ public class ProtobufSchemaCodegen extends DefaultCodegen implements CodegenConf
      * @param allowableValues allowable values
      */
     public void addUnspecifiedToAllowableValues(Map<String, Object> allowableValues) {
+
+        final String UNSPECIFIED = "UNSPECIFIED";
+
         if (startEnumsWithUnspecified) {
             if (allowableValues.containsKey("enumVars")) {
                 List<Map<String, Object>> enumVars = (List<Map<String, Object>>) allowableValues.get("enumVars");
-
-                HashMap<String, Object> unspecified = new HashMap<String, Object>();
-                unspecified.put("name", "UNSPECIFIED");
-                unspecified.put("isString", "false");
-                unspecified.put("value", "\"UNSPECIFIED\"");
-                enumVars.add(0, unspecified);
+                boolean unspecifiedPresent = enumVars.stream()
+                        .anyMatch(e -> {
+                            return UNSPECIFIED.equals(e.get("name"));
+                        });
+                if (!unspecifiedPresent) {
+                    HashMap<String, Object> unspecifiedEnum = new HashMap<String, Object>();
+                    unspecifiedEnum.put("name", UNSPECIFIED);
+                    unspecifiedEnum.put("isString", "false");
+                    unspecifiedEnum.put("value", "\"" + UNSPECIFIED + "\"");
+                    enumVars.add(0, unspecifiedEnum);
+                }
             }
 
             if (allowableValues.containsKey("values")) {
                 List<String> values = (List<String>) allowableValues.get("values");
-                List<String> modifiableValues = new ArrayList<>(values);
-                modifiableValues.add(0, "UNSPECIFIED");
-                allowableValues.put("values", modifiableValues);
+                if (!values.contains(UNSPECIFIED)) {
+                    List<String> modifiableValues = new ArrayList<>(values);
+                    modifiableValues.add(0, UNSPECIFIED);
+                    allowableValues.put("values", modifiableValues);
+                }
             }
         }
     }
diff --git a/modules/openapi-generator/src/test/java/org/openapitools/codegen/protobuf/ProtobufSchemaCodegenTest.java b/modules/openapi-generator/src/test/java/org/openapitools/codegen/protobuf/ProtobufSchemaCodegenTest.java
index 68fe376efca..573abc3124b 100644
--- a/modules/openapi-generator/src/test/java/org/openapitools/codegen/protobuf/ProtobufSchemaCodegenTest.java
+++ b/modules/openapi-generator/src/test/java/org/openapitools/codegen/protobuf/ProtobufSchemaCodegenTest.java
@@ -45,6 +45,7 @@ import java.util.Map;
 import static org.openapitools.codegen.TestUtils.createCodegenModelWrapper;
 import static org.openapitools.codegen.languages.ProtobufSchemaCodegen.USE_SIMPLIFIED_ENUM_NAMES;
 import static org.testng.Assert.assertEquals;
+import static org.openapitools.codegen.languages.ProtobufSchemaCodegen.START_ENUMS_WITH_UNSPECIFIED;
 
 public class ProtobufSchemaCodegenTest {
 
@@ -213,4 +214,86 @@ public class ProtobufSchemaCodegenTest {
         Assert.assertEquals(enumVars2.get(1).get("value"), simpleEnumValue ? "_2" : "\"TEST_INT_ENUM__2\"");
         Assert.assertEquals(enumVars2.get(1).get("isString"), false);
     }
+
+    @SuppressWarnings("unchecked")
+    @Test(description = "Validate that unspecified enum values are added when the option is selected")
+    public void unspecifiedEnumValuesAreAdded() {
+        String enumKey = "aValidEnumWithoutUnspecifiedValues";
+
+        final Schema<?> model = new Schema<>()
+                .description("a sample model")
+                .addProperty(enumKey, new StringSchema()._enum(Arrays.asList("foo", "bar")));
+
+        final ProtobufSchemaCodegen codegen = new ProtobufSchemaCodegen();
+        OpenAPI openAPI = TestUtils.createOpenAPIWithOneSchema("sample", model);
+        codegen.setOpenAPI(openAPI);
+        final CodegenModel cm = codegen.fromModel("sample", model);
+        codegen.additionalProperties().put(USE_SIMPLIFIED_ENUM_NAMES, true);
+        codegen.additionalProperties().put(START_ENUMS_WITH_UNSPECIFIED, true);
+
+        codegen.processOpts();
+        codegen.postProcessModels(createCodegenModelWrapper(cm));
+
+        final CodegenProperty property1 = cm.vars.get(0);
+        Assert.assertEquals(property1.baseName, enumKey);
+        Assert.assertEquals(property1.dataType, "string");
+        Assert.assertEquals(property1.baseType, "string");
+        Assert.assertEquals(property1.datatypeWithEnum, "A_valid_enum_without_unspecified_values");
+        Assert.assertEquals(property1.name, "a_valid_enum_without_unspecified_values");
+        Assert.assertTrue(property1.isEnum);
+        Assert.assertEquals(property1.allowableValues.size(), 2);
+        List<Map<String, Object>> enumVars1 = (List<Map<String, Object>>) property1.allowableValues.get("enumVars");
+        Assert.assertEquals(enumVars1.size(), 3);
+
+        Assert.assertEquals(enumVars1.get(0).get("name"), "UNSPECIFIED");
+        Assert.assertEquals(enumVars1.get(0).get("value"), "UNSPECIFIED");
+        Assert.assertEquals(Boolean.valueOf((String) enumVars1.get(0).get("isString")), false);
+
+        Assert.assertEquals(enumVars1.get(1).get("name"), "FOO");
+        Assert.assertEquals(enumVars1.get(1).get("value"), "FOO");
+        Assert.assertEquals(enumVars1.get(1).get("isString"), false);
+
+        Assert.assertEquals(enumVars1.get(2).get("name"), "BAR");
+        Assert.assertEquals(enumVars1.get(2).get("value"), "BAR");
+        Assert.assertEquals(enumVars1.get(2).get("isString"), false);
+    }
+
+    @SuppressWarnings("unchecked")
+    @Test(description = "Validate that unspecified enum values are NOT added when the option is selected if they are already present")
+    public void unspecifiedEnumValuesIgnoredIfAlreadyPresent() {
+        String enumKey = "aValidEnumWithUnspecifiedValues";
+
+        final Schema<?> model = new Schema<>()
+                .description("a sample model")
+                .addProperty(enumKey, new StringSchema()._enum(Arrays.asList( "UNSPECIFIED", "foo")));
+
+        final ProtobufSchemaCodegen codegen = new ProtobufSchemaCodegen();
+        OpenAPI openAPI = TestUtils.createOpenAPIWithOneSchema("sample", model);
+        codegen.setOpenAPI(openAPI);
+        final CodegenModel cm = codegen.fromModel("sample", model);
+        codegen.additionalProperties().put(USE_SIMPLIFIED_ENUM_NAMES, true);
+        codegen.additionalProperties().put(START_ENUMS_WITH_UNSPECIFIED, true);
+
+        codegen.processOpts();
+        codegen.postProcessModels(createCodegenModelWrapper(cm));
+
+        final CodegenProperty property1 = cm.vars.get(0);
+        Assert.assertEquals(property1.baseName, enumKey);
+        Assert.assertEquals(property1.dataType, "string");
+        Assert.assertEquals(property1.baseType, "string");
+        Assert.assertEquals(property1.datatypeWithEnum, "A_valid_enum_with_unspecified_values");
+        Assert.assertEquals(property1.name, "a_valid_enum_with_unspecified_values");
+        Assert.assertTrue(property1.isEnum);
+        Assert.assertEquals(property1.allowableValues.size(), 2);
+        List<Map<String, Object>> enumVars1 = (List<Map<String, Object>>) property1.allowableValues.get("enumVars");
+        Assert.assertEquals(enumVars1.size(), 2);
+
+        Assert.assertEquals(enumVars1.get(0).get("name"), "UNSPECIFIED");
+        Assert.assertEquals(enumVars1.get(0).get("value"), "UNSPECIFIED");
+        Assert.assertEquals(enumVars1.get(0).get("isString"), false);
+
+        Assert.assertEquals(enumVars1.get(1).get("name"), "FOO");
+        Assert.assertEquals(enumVars1.get(1).get("value"), "FOO");
+        Assert.assertEquals(enumVars1.get(1).get("isString"), false);
+    }
 }