prometheus-operator CRDs cause problems when re-running ansible
When you re-install prometheus, you first need to remove these CRDs:
$ kubectl delete crd servicemonitors.monitoring.coreos.com prometheusrules.monitoring.coreos.com prometheuses.monitoring.coreos.com podmonitors.monitoring.coreos.com alertmanagers.monitoring.coreos.com
Otherwise, you'll run into Kubernetes complaining that they already exist.
A possible solution is to manually make the CRDs and disable making them in the helmfile values. However, another possibility is to add the variable that disables making the CRDs to the cluster's local override values after installing prometheus-operator.
Related:
https://github.com/helm/charts/tree/master/stable/prometheus-operator#helm-fails-to-create-crds