Test error handling for re-use of a VDP filter name.

src/tests/vdp.vtc

@@ -169,6 +169,15 @@ varnish v1 -errvcl {vdp pipe failure: new enoent: cannot execute ${tmpdir}/foo:
 	}
 }
 
+varnish v1 -errvcl {vdp pipe failure: new taken: filter name esi already in use by another VDP} {
+	import ${vmod_pipe};
+	backend b None;
+
+	sub vcl_init {
+		new taken = pipe.vdp(path="${cat}", name="esi");
+	}
+}
+
 varnish v1 -vcl { backend b None; }
 
 # Tests the discard event, with removal of VDPs.