Description

Dispatch of a regular message may be paused when working with custom tokens because it is not thread-safe. The following error can be found in the log records:

MessageTaskRunner worker thread 7 
ERROR Message sending error: System.ArgumentException: An item with the same key has already been added.
   at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
   at Sitecore.Modules.EmailCampaign.Core.Dispatch.DispatchManager.RestoreCustomTokensFromQueue(MessageItem message, Dictionary`2 customPersonTokens)
   at Sitecore.Modules.EmailCampaign.Core.Dispatch.DispatchTask.GenerateEmailContent(Recipient recipient, String emailAddress, Language language, Guid recipientId, Dictionary`2 customPersonTokens, DateTime& startGetPageTime, DateTime& endGetPageTime, DateTime& startCollectFilesTime, DateTime& endCollectFilesTime)
   at Sitecore.Modules.EmailCampaign.Core.Dispatch.DispatchTask.OnSendToNextRecipient() 

This issue affects EXM 3.2 Update-1 - 3.4 Update-2 versions.

Solution

To resolve the issue consider one of the following options:

• Upgrade Sitecore XP instance to 9.0 Update-1 or higher (recommended).
• Download and install the patch compatible with the affected product version found on this page:
  https://github.com/SitecoreSupport/Sitecore.Support.151324/releases.