enhance unused (#3800)

lib/compress.js

@@ -4200,8 +4200,15 @@ merge(Compressor.prototype, {
                     } else if (sym.orig[0] instanceof AST_SymbolCatch) {
                         var value = def.value && def.value.drop_side_effect_free(compressor);
                         if (value) side_effects.push(value);
-                        def.value = null;
+                        var var_defs = var_defs_by_id.get(sym.id);
-                        head.push(def);
+                        if (var_defs.length > 1) {
+                            AST_Node.warn("Dropping duplicated declaration of variable {name} [{file}:{line},{col}]", template(def.name));
+                            remove(var_defs, def);
+                            sym.eliminated++;
+                        } else {
+                            def.value = null;
+                            head.push(def);
+                        }
                     } else {
                         var value = def.value && !def.value.single_use && def.value.drop_side_effect_free(compressor);
                         if (value) {

test/compress/drop-unused.js

@@ -1173,7 +1173,6 @@ var_catch_toplevel: {
                 x();
             } catch (a) {
                 var a;
-                var a;
             }
         }();
     }
@@ -2465,3 +2464,27 @@ drop_duplicated_side_effects: {
     }
     expect_stdout: "1"
 }
+
+drop_duplicated_var_catch: {
+    options = {
+        unused: true,
+    }
+    input: {
+        function f() {
+            try {
+                x();
+            } catch (a) {
+                var a, a;
+            }
+        }
+    }
+    expect: {
+        function f() {
+            try {
+                x();
+            } catch (a) {
+                var a;
+            }
+        }
+    }
+}