diff --git a/lib/parse.js b/lib/parse.js
index 5b52d6d16..c99016a08 100644
--- a/lib/parse.js
+++ b/lib/parse.js
@@ -1572,66 +1572,66 @@ function parse($TEXT, options) {
         });
     };
 
-    function track_used_binding_identifiers(is_parameter, strict) {
-        var parameters = new Set();
-        var duplicate = false;
-        var default_assignment = false;
-        var spread = false;
-        var strict_mode = !!strict;
-        var tracker = {
-            add_parameter: function(token) {
-                if (parameters.has(token.value)) {
-                    if (duplicate === false) {
-                        duplicate = token;
-                    }
-                    tracker.check_strict();
-                } else {
-                    parameters.add(token.value);
-                    if (is_parameter) {
-                        switch (token.value) {
-                          case "arguments":
-                          case "eval":
-                          case "yield":
-                            if (strict_mode) {
-                                token_error(token, "Unexpected " + token.value + " identifier as parameter inside strict mode");
-                            }
-                            break;
-                          default:
-                            if (RESERVED_WORDS.has(token.value)) {
-                                unexpected();
-                            }
+    class UsedParametersTracker {
+        constructor(is_parameter, strict, duplicates_ok = false) {
+            this.is_parameter = is_parameter;
+            this.duplicates_ok = duplicates_ok;
+            this.parameters = new Set();
+            this.duplicate = null;
+            this.default_assignment = false;
+            this.spread = false;
+            this.strict_mode = !!strict;
+        }
+        add_parameter(token) {
+            if (this.parameters.has(token.value)) {
+                if (this.duplicate === null) {
+                    this.duplicate = token;
+                }
+                this.check_strict();
+            } else {
+                this.parameters.add(token.value);
+                if (this.is_parameter) {
+                    switch (token.value) {
+                      case "arguments":
+                      case "eval":
+                      case "yield":
+                        if (this.strict_mode) {
+                            token_error(token, "Unexpected " + token.value + " identifier as parameter inside strict mode");
+                        }
+                        break;
+                      default:
+                        if (RESERVED_WORDS.has(token.value)) {
+                            unexpected();
                         }
                     }
                 }
-            },
-            mark_default_assignment: function(token) {
-                if (default_assignment === false) {
-                    default_assignment = token;
-                }
-            },
-            mark_spread: function(token) {
-                if (spread === false) {
-                    spread = token;
-                }
-            },
-            mark_strict_mode: function() {
-                strict_mode = true;
-            },
-            is_strict: function() {
-                return default_assignment !== false || spread !== false || strict_mode;
-            },
-            check_strict: function() {
-                if (tracker.is_strict() && duplicate !== false) {
-                    token_error(duplicate, "Parameter " + duplicate.value + " was used already");
-                }
             }
-        };
-
-        return tracker;
+        }
+        mark_default_assignment(token) {
+            if (this.default_assignment === false) {
+                this.default_assignment = token;
+            }
+        }
+        mark_spread(token) {
+            if (this.spread === false) {
+                this.spread = token;
+            }
+        }
+        mark_strict_mode() {
+            this.strict_mode = true;
+        }
+        is_strict() {
+            return this.default_assignment !== false || this.spread !== false || this.strict_mode;
+        }
+        check_strict() {
+            if (this.is_strict() && this.duplicate !== null && !this.duplicates_ok) {
+                token_error(this.duplicate, "Parameter " + this.duplicate.value + " was used already");
+            }
+        }
     }
 
     function parameters(params) {
-        var used_parameters = track_used_binding_identifiers(true, S.input.has_directive("use strict"));
+        var used_parameters = new UsedParametersTracker(true, S.input.has_directive("use strict"));
 
         expect("(");
 
@@ -1655,7 +1655,7 @@ function parse($TEXT, options) {
         var param;
         var expand = false;
         if (used_parameters === undefined) {
-            used_parameters = track_used_binding_identifiers(true, S.input.has_directive("use strict"));
+            used_parameters = new UsedParametersTracker(true, S.input.has_directive("use strict"));
         }
         if (is("expand", "...")) {
             expand = S.token;
@@ -1698,7 +1698,9 @@ function parse($TEXT, options) {
         var expand_token;
         var first_token = S.token;
         if (used_parameters === undefined) {
-            used_parameters = track_used_binding_identifiers(false, S.input.has_directive("use strict"));
+            const strict = S.input.has_directive("use strict");
+            const duplicates_ok = symbol_type === AST_SymbolVar;
+            used_parameters = new UsedParametersTracker(false, strict, duplicates_ok);
         }
         symbol_type = symbol_type === undefined ? AST_SymbolFunarg : symbol_type;
         if (is("punc", "[")) {
@@ -2091,7 +2093,7 @@ function parse($TEXT, options) {
             if (is("punc", "{") || is("punc", "[")) {
                 def = new AST_VarDef({
                     start: S.token,
-                    name: binding_element(undefined ,sym_type),
+                    name: binding_element(undefined, sym_type),
                     value: is("operator", "=") ? (expect_token("operator", "="), expression(false, no_in)) : null,
                     end: prev()
                 });