re-add no-duplicate-selectors

Author: nadalaba

frontend/static/themes/rainbow_trail.css

@@ -37,7 +37,6 @@ footer button:hover,
 footer button:hover > i,
 footer button:hover > .relative > i,
 #restartTestButton:hover > i,
-#restartTestButton:hover > i,
 #showWordHistoryButton:hover > i,
 #saveScreenshotButton:hover > i,
 #restartTestButtonWithSameWordset:hover > i,

stylelint.config.mjs

@@ -2,8 +2,6 @@
 export default {
   extends: ["stylelint-config-standard"],
   rules: {
-    "no-duplicate-selectors": null, // add
-
     "font-family-no-missing-generic-family-keyword": null, // add
 
     "declaration-block-no-shorthand-property-overrides": null, // add