Why mongorestore return error Duplicated _id in timeseries.bucket?

I want to migrate data from regular collection to timeseries collection, I am following this instruction: https://www.mongodb.com/docs/manual/core/timeseries/timeseries-migrate-data-into-timeseries-collection/

Step 1: Create a new timeseries collection (weather_timeseries)
Step 2: Use aggregate with $out operation to transform data to timeseries structure, store to weather_temporary collection
Step 3: Use mongo dump to export data from weather_temporary to weather_temporary.bson file
Step 4: Use mongo restore to import data from weather_temporary.bson to weather_timeseries collection

Step 1, 2, 3 works well. The problem in step 4.
It ran about 11% then show a error and stop processing.

Error message: Failed: test.weather_timeseries: error restoring from timeseries/test/weather_timeseries.bson: bulk write exception: write errors: [E11000 duplicate key error collection: test.system.buckets.weather_timeseries dup key: { _id: ObjectId(‘xxxxxxxxx’) }]

as I know, test.system.buckets is system collection has managed by mongo, each data insert to weather_timeseries, the data will be aggregated into system.buckets. And _id in buckets is generated by mongo as well. I don’t know how this error can be happened