fix corner case in unused (#4767)

fixes #4766

lib/compress.js

@@ -6216,7 +6216,10 @@ merge(Compressor.prototype, {
                             });
                             body.push(defun);
                         } else {
-                            if (drop_sym && var_defs[sym.id] > 1 && sym.orig.indexOf(def.name) > sym.eliminated) {
+                            if (drop_sym
+                                && var_defs[sym.id] > 1
+                                && !(parent instanceof AST_ExportDeclaration)
+                                && sym.orig.indexOf(def.name) > sym.eliminated) {
                                 var_defs[sym.id]--;
                                 duplicated++;
                             }

test/compress/exports.js

@@ -464,3 +464,17 @@ issue_4761: {
     }
     expect_exact: 'export default"function"==42;'
 }
+
+issue_4766: {
+    options = {
+        unused: true,
+    }
+    input: {
+        var a = "foo";
+        export var a = "bar";
+    }
+    expect: {
+        var a = "foo";
+        export var a = "bar";
+    }
+}