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Concurrently joining control-plane nodes does not work as expected in kubeadm 1.15.0. The feature was planned for release in 1.15.0, but a fix may  come in a follow up patch release.

- Using `--log-file` is known to be problematic in 1.15. This presents as things being logged multiple times to the same file. The behaviour and details of this issue, as well as some preliminary attempts at fixing it are documented [here](https://github.com/kubernetes/kubernetes/issues/78734#issuecomment-501372131)...