When we return
{ mustRetry: true, options: options }
and the options has agent property in our own custom retry strategy, we got ERR_MULTIPLE_CALLBACK.

The same code, if we just return the bool value of mustRetry or we delete the agent property in options, then it works fine as before. It seems the agent is NOT cloneable.

Env:
nodejs: 12.19.0
request: 2.88.1
reqeustretry: 4.1.1
agentkeepalive: 4.1.3