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Hi @Kendra,
Thank you for taking the time to explain what you've already tried.
Based on your description, this appears to be a Power Automate workflow behavior issue involving the interaction between Outlook triggers, SharePoint validation logic, and Planner task creation, rather than an issue with SharePoint or Microsoft 365 core services themselves.
Since your question is currently posted under Microsoft 365 and Office | SharePoint, I'm concerned that you may not receive the most specialized guidance for this scenario here. Troubleshooting flow trigger behavior, duplicate executions, and flow design logic typically falls within the scope of Power Automate (Power Platform) expertise.
To help you get the most accurate assistance, I'd recommend posting your question in the Microsoft Power Automate Community, where Power Automate specialists, MVPs, and experienced community members regularly assist with flow design and troubleshooting.
If possible, include screenshots of your flow (with sensitive information removed), particularly the trigger, SharePoint lookup/check logic, and Planner task creation actions, along with a sample run history showing the duplicate entries.
I apologize for pointing you to another community, but my goal is simply to help you reach the people who are most likely to have hands-on experience with this specific type of automation issue. That will probably save you time and lead to a resolution more quickly than trying to troubleshoot it here.
Thank you for your understanding, and I hope you're able to get this resolved quickly.