BUG/MINOR: dns: check for duplicate nameserver id in a resolvers section was missing

Current resolvers section parsing function is permissive on nameserver
id and two nameservers may have the same id.
It's a shame, since we don't know for example, whose statistics belong
to which nameserver...

From now, configuration with duplicated nameserver id in a resolvers
section are considered as broken and returns a fatal error when parsing.
diff --git a/src/cfgparse.c b/src/cfgparse.c
index 610e165..f4bc503 100644
--- a/src/cfgparse.c
+++ b/src/cfgparse.c
@@ -2262,6 +2262,15 @@
 			goto out;
 		}
 
+		list_for_each_entry(newnameserver, &curr_resolvers->nameserver_list, list) {
+			/* Error if two resolvers owns the same name */
+			if (strcmp(newnameserver->id, args[1]) == 0) {
+				Alert("Parsing [%s:%d]: nameserver '%s' has same name as another nameserver (declared at %s:%d).\n",
+					file, linenum, args[1], curr_resolvers->conf.file, curr_resolvers->conf.line);
+				err_code |= ERR_ALERT | ERR_FATAL;
+			}
+		}
+
 		if ((newnameserver = (struct dns_nameserver *)calloc(1, sizeof(struct dns_nameserver))) == NULL) {
 			Alert("parsing [%s:%d] : out of memory.\n", file, linenum);
 			err_code |= ERR_ALERT | ERR_ABORT;