[Minor] Fix non-conformant config file

conf/modules.d/dmarc.conf

@@ -15,6 +15,6 @@
 
 dmarc {
     .include(try=true,priority=5) "${DBDIR}/dynamic/dmarc.conf"
-    .include(try=true,priority=1) "$LOCAL_CONFDIR/local.d/dmarc.conf"
+    .include(try=true,priority=1,duplicate=merge) "$LOCAL_CONFDIR/local.d/dmarc.conf"
     .include(try=true,priority=10) "$LOCAL_CONFDIR/override.d/dmarc.conf"
 }