Hi,
This migt be related to the rules. It seems as if two rules are fired sending autoacknowledge E-Mails and you did not include an action stopping further rule processing.
Do you have more than one rule which results in sending an E-Mail?
Another possibility beside debugging is checking in the IC_MANAGER role to see which rules have been evaluated to TRUE which helps you to review the logic of the rule policy.
Best Regards,
Sigrid