If you have OpacLangSelectorMode set to "both" the page will have a duplicate id because the same menu include is used in the header and the footer.
Created attachment 185229 [details] [review] Bug 40612: Eliminate duplicate element id in OPAC language menus This patch updates the include file used to display the OPAC language menu in the header and footer. The include already makes use of a "context" variable which tells it whether it's in the header or footer. We can use the same logic to give unique ids for each instance. To test, apply the patch and rebuild the OPAC CSS. - Go to Administration -> System preferences -> OpacLangSelectorMode and set it to "both top and footer." - Go to the OPAC and inspect the HTML for each language menu. The one in the header should have the id "header_langmenu." The one in the footer should have "footer_langmenu." Sponsored-by: Athens County Public Libraries