In http-proxy-middleware before 2.0.8 and 3.x before 3.0.4, writeBody can be called twice because "else if" is not used.
https://github.com/chimurai/http-proxy-middleware/releases/tag/v3.0.4
https://github.com/chimurai/http-proxy-middleware/releases/tag/v2.0.8
https://github.com/chimurai/http-proxy-middleware/pull/1089
https://github.com/chimurai/http-proxy-middleware/commit/020976044d113fc0bcbbaf995e91d05e2829a145