don't insert unused variables in the SingleFile stylesheet

lib/single-file/single-file-core.js

@@ -1027,10 +1027,9 @@ this.SingleFileCore = this.SingleFileCore || (() => {
 						urlFunction = "url(" + JSON.stringify(resourceURL) + ")";
 						const regExpUrlFunction = DomUtil.getRegExp(urlFunction);
 						if (duplicate && options.groupDuplicateImages) {
-							resourceInfos.set(resourceURL, { regExpUrlFunction, dataURI: content, variableName: "var(" + SINGLE_FILE_VARIABLE_NAME_PREFIX + indexResource + ")" });
-							DomUtil.insertVariable(doc, indexResource, content, options);
+							resourceInfos.set(resourceURL, { regExpUrlFunction, indexResource, dataURI: content, variableName: "var(" + SINGLE_FILE_VARIABLE_NAME_PREFIX + indexResource + ")" });
 						} else {
-							resourceInfos.set(resourceURL, { regExpUrlFunction, dataURI: content });
+							resourceInfos.set(resourceURL, { regExpUrlFunction, indexResource, dataURI: content });
 						}
 					}
 				}
@@ -1050,6 +1049,7 @@ this.SingleFileCore = this.SingleFileCore || (() => {
 					} else if (cssRule.type == CSSRule.STYLE_RULE) {
 						rulesContent += processURLFunctions(cssRule.cssText);
 					} else {
+
 						rulesContent += processURLFunctions(cssRule.cssText, true);
 					}
 				});
@@ -1066,6 +1066,7 @@ this.SingleFileCore = this.SingleFileCore || (() => {
 							const resourceInfo = resourceInfos.get(resourceURL);
 							if (options.groupDuplicateImages && resourceInfo.variableName && !preventGrouping) {
 								cssText = cssText.replace(resourceInfo.regExpUrlFunction, resourceInfo.variableName);
+								DomUtil.insertVariable(doc, resourceInfo.indexResource, resourceInfo.dataURI, options);
 							} else {
 								cssText = cssText.replace(resourceInfo.regExpUrlFunction, urlFunction.replace(originalResourceURL, resourceInfo.dataURI));
 							}