No need to append the zipflow filter

src/vtc/sub-body.vtc

@@ -19,7 +19,7 @@ varnish v1 -vcl {
 		synthetic(" /FIRST/FROM/RESP/file /file1");
 		zipflow.subreqs_from_body(resp_body);
 		zipflow.subreqs_from_body(req_body);
-		set resp.filters += " zipflow";
+		set resp.filters = "zipflow";
 	}
 
 	sub synth_sub {

src/vtc/sub-coalesce.vtc

@@ -28,7 +28,7 @@ varnish v1 -vcl+backend {
 		zipflow.meta(name="top");
 		zipflow.subreq("/file1");
 		zipflow.subreq("/file2");
-		set resp.filters += " zipflow";
+		set resp.filters = "zipflow";
 		return (deliver);
 	}
 } -start

src/vtc/sub.vtc

@@ -17,7 +17,7 @@ varnish v1 -vcl {
 		zipflow.meta(name="top");
 		zipflow.subreq("/file1");
 		zipflow.subreq("/file2");
-		set resp.filters += " zipflow";
+		set resp.filters = "zipflow";
 	}
 
 	sub synth_sub {

src/vtc/vmod_zipflow.vtc

@@ -30,7 +30,7 @@ varnish v1 -vcl+backend {
 		if (req.http.mtime) {
 			zipflow.meta(mtime=std.time(req.http.mtime));
 		}
-		set resp.filters += " zipflow";
+		set resp.filters = "zipflow";
 	}
 } -start
 

src/vtc/zero.vtc

@@ -13,7 +13,7 @@ varnish v1 -vcl {
 		if (zipflow.is_subreq()) {
 			synthetic("file content");
 		} else {
-			set resp.filters += " zipflow";
+			set resp.filters = "zipflow";
 			zipflow.subreq("/file");
 			synthetic("");
 		}