An item with the same key has already been added when updating a record

I have a workflow with 2 steps:

1. Get Record Guid <-- Custom Activity

2. Write the Record Guid into a field (Standard Update Record Step)

I am importing a lot of data and sometimes got the error:

[Microsoft.Crm.ObjectModel: Microsoft.Crm.ObjectModel.SyncWorkflowExecutionPlugin]
[1f879836-d8d0-e511-80dd-5065f38be311: ]
Starting sync workflow 'Participation set RecordID', Id: 19879836-d8d0-e511-80dd-5065f38be311
Entering CustomActivityStep3_step: Get RecordID
Entering UpdateStep2_step:
Sync workflow 'Participation set RecordID' terminated with error 'An item with the same key has already been added.'

Any help is appreciated.

Thanks.