check duplicate resolver

src/mail/ngx_mail_core_module.c

diff --git a/src/mail/ngx_mail_core_module.c b/src/mail/ngx_mail_core_module.c
index 15627d2..03635fc 100644
--- a/src/mail/ngx_mail_core_module.c
+++ b/src/mail/ngx_mail_core_module.c
@@ -422,6 +422,10 @@
 
     value = cf->args->elts;
 
+    if (cscf->resolver != NGX_CONF_UNSET_PTR) {
+        return "is duplicate";
+    }
+
     if (ngx_strcmp(value[1].data, "off") == 0) {
         cscf->resolver = NULL;
         return NGX_CONF_OK;