Received duplicated jobs

This is the steps i took.
1. Start disque-server with one node only.
2. Adding 10000 jobs with RETRY 300 seconds to the que_0 one by one in a single thread. When a job is added, a record(job id, create time) is inserted in the mysql db.
3. Another process get the jobs from the que_0 one by one in a single thread.
When a job is gotten, The sql is executed:
update queue set executeStartTime = :now, executeStartTimeStr = CONCAT(executeStartTimeStr, :nowStr) where jobId = :jobId

Then the job will be acked, the sql executed:
update queue set ack = ack + 1, endTime = :now, executeEndTimeStr = CONCAT(executeEndTimeStr, :nowStr) where jobId = :jobId
The time between the job is got and acked is less than 30 seconds.
4. After all the 10000 jobs are added and gotted, the result is:
9997 jobs with one time and 3 duplicated jobs got.

select \* from queue where ack=2, got the following results:

5.The disque client i used is spinach