diff --git a/argparse.js b/argparse.js
index 2b8c8c6..0999937 100644
--- a/argparse.js
+++ b/argparse.js
@@ -1080,6 +1080,13 @@ const ArgumentDefaultsHelpFormatter = _camelcase_alias(_callable(class ArgumentD
 
     _get_help_string(action) {
         let help = action.help
+
+        // If the action already appended its own default argument description,
+        // then don't attempt to caculate an additional or new one.
+        if (action.has_default_help_hint === true) {
+            return help
+        }
+
         // LEGACY (v1 compatibility): additional check for defaultValue needed
         if (!action.help.includes('%(default)') && !action.help.includes('%(defaultValue)')) {
             if (action.default !== SUPPRESS) {
@@ -1265,6 +1272,7 @@ const Action = _camelcase_alias(_callable(class Action extends _AttributeHolder(
         this.required = required
         this.help = help
         this.metavar = metavar
+        this.has_default_help_hint = false
     }
 
     _get_kwargs() {
@@ -1325,8 +1333,10 @@ const BooleanOptionalAction = _camelcase_alias(_callable(class BooleanOptionalAc
             }
         }
 
+        let did_appened_default_help_desc = false
         if (help !== undefined && default_value !== undefined) {
             help += ` (default: ${default_value})`
+            did_appened_default_help_desc = true
         }
 
         super({
@@ -1340,6 +1350,8 @@ const BooleanOptionalAction = _camelcase_alias(_callable(class BooleanOptionalAc
             help,
             metavar
         })
+
+        this.has_default_help_hint = did_appened_default_help_desc
     }
 
     call(parser, namespace, values, option_string = undefined) {
diff --git a/test/test_argparse.js b/test/test_argparse.js
index 0a0b348..131afd0 100644
--- a/test/test_argparse.js
+++ b/test/test_argparse.js
@@ -845,6 +845,29 @@ class ParserTestCase extends TestCase {
     }
 }).run()
 
+;(new class TestBooleanOptionalActionHelpWithArgumentDefaultsHelpFormatter extends TestCase {
+    // Test that using BooleanOptionalAction with ArgumentDefaultsHelpFormatter
+    // doesn't result in duplicate '(default: <value>)' suffixes.
+
+    // See https://github.com/nodeca/argparse/issues/177
+
+    test_no_duplicate_default_desc() {
+        const parser = argparse.ArgumentParser({
+            formatter_class: argparse.ArgumentDefaultsHelpFormatter,
+        })
+        parser.add_argument('-e', '--an-example', {
+            help: 'Example',
+            action: argparse.BooleanOptionalAction,
+            default: true
+        })
+        const parser_help = parser.format_help()
+        const default_regex_rs = parser_help.match(/\(default: /g)
+        this.assertNotEqual(default_regex_rs, null)
+        const num_default_arg_descriptions = default_regex_rs.length
+        this.assertEqual(num_default_arg_descriptions, 1)
+    }
+}).run()
+
 ;(new class TestBooleanOptionalActionRequired extends ParserTestCase {
     /* Tests BooleanOptionalAction required */