forwardauth: error on duplicate uri subdirective (#7814)

Author: vijayvenkatj

caddytest/integration/caddyfile_adapt/forward_auth_duplicate_uri.caddyfiletest

@@ -0,0 +1,12 @@
+localhost:9080 {
+	forward_auth :9091 {
+		uri /first
+		uri /second
+		copy_headers Remote-User
+	}
+
+	respond "ok" 200
+}
+
+----------
+parsing caddyfile tokens for 'forward_auth': cannot re-declare uri: /second

modules/caddyhttp/reverseproxy/forwardauth/caddyfile.go

@@ -129,7 +129,11 @@ func parseCaddyfile(h httpcaddyfile.Helper) ([]httpcaddyfile.ConfigValue, error)
 				if !dispenser.NextArg() {
 					return nil, dispenser.ArgErr()
 				}
-				rpHandler.Rewrite.URI = dispenser.Val()
+				uri := dispenser.Val()
+				if rpHandler.Rewrite.URI != "" {
+					return nil, dispenser.Errf("cannot re-declare uri: %s", uri)
+				}
+				rpHandler.Rewrite.URI = uri
 				dispenser.DeleteN(2)
 
 			case "copy_headers":