GUACAMOLE-1757: Ensure SSO provider list is added to login UI only once.

extensions/guacamole-auth-sso/modules/guacamole-auth-sso-base/src/main/resources/html/sso-providers.html

@@ -1,4 +1,4 @@
-<meta name="after" content=".login-ui .login-dialog-middle">
+<meta name="after" content=".login-ui .login-dialog-middle:not(:has(~ .sso-providers))">
 <div class="sso-providers">
     {{ 'LOGIN.SECTION_HEADER_SSO_OPTIONS' | translate }}
     <ul class="sso-provider-list"></ul>