Sync error: "Mongo::Error::OperationFailure: E11000 duplicate key error collection"

[katherine-boost]

When trying to run rake consyncful:refresh, I was consistently seeing this error,

syncing: H4n7aK3F48m0FTXe6g65R
syncing: YOLww2jgz5Wx8mi6UQzkr
rake aborted!
Mongo::Error::OperationFailure: E11000 duplicate key error collection: pvm_rails_development.contentful_models index: _id_ dup key: { _id: "YOLww2jgz5Wx8mi6UQzkr" } (11000) (on localhost:27017, legacy retry, attempt 1)

Tasks: TOP => consyncful:refresh
(See full trace by running task with --trace)