Long task fire multi times

I test a long task:
class AClass {
public void Hahaha(int i) {
_logger.LogError("Message from AClass.hahaha begin " + i.ToString());
Thread.Sleep(5000);
_logger.LogError("Message from AClass.hahaha end " + i.ToString());
}
}
BackgroundJob.Enqueue(c => c.Hahaha(t))

then I got this:

2018-06-08 15:54:44,273 ERROR [Worker #81645ab9] ?.? - Message from AClass.hahaha begin 1682564419
2018-06-08 15:54:46,086 ERROR [Worker #3317f49e] ?.? - Message from AClass.hahaha begin 1682564419
2018-06-08 15:54:48,084 ERROR [Worker #7bd544a0] ?.? - Message from AClass.hahaha begin 1682564419
2018-06-08 15:54:49,279 ERROR [Worker #81645ab9] ?.? - Message from AClass.hahaha end 1682564419
2018-06-08 15:54:51,093 ERROR [Worker #3317f49e] ?.? - Message from AClass.hahaha end 1682564419
2018-06-08 15:54:53,089 ERROR [Worker #7bd544a0] ?.? - Message from AClass.hahaha end 1682564419

My config:
GlobalConfiguration.Configuration.UseStorage(
new MySqlStorage(GetMysqlConnectionString(), new MySqlStorageOptions
{
TransactionIsolationLevel = IsolationLevel.ReadCommitted,
QueuePollInterval = TimeSpan.FromSeconds(15),
JobExpirationCheckInterval = TimeSpan.FromHours(1),
CountersAggregateInterval = TimeSpan.FromMinutes(5),
PrepareSchemaIfNecessary = true,
DashboardJobListLimit = 50000,
TransactionTimeout = TimeSpan.FromMinutes(1),
}));
How to avoid this?

Reproduce the behavior with the shown AClass.Hahaha method and BackgroundJob.Enqueue call, then trace how the configured MySqlStorage and worker polling handle the long-running job. Check whether the repeated starts are expected retries or duplicate processing; done should establish the cause and provide a verified way to prevent unintended multiple executions.

Written by the indexing model from the issue text.