Azure function with Timer Trigger firing twice

@MatheusNani Thanks for reaching out. There could be different reasons for this issue, and one of the reasons documented here. I will suggest you to review the document below to troubleshoot the issue and see if you are able to find the root cause.

Timer triggered function app uses TimerTriggerAttribute. This attribute consists of the Singleton Lock feature which ensures that only a single instance of the function is running at any given time. If any process runs longer than the scheduled timer, the new incoming process waits for the older process to finish and then uses the same instance. If you are using the same storage account across different timer trigger functions then this could be one of the reasons as mentioned here.
You can disable that behavior if you wish by setting UseMonitor = false on your TimerTrigger attribute and see if resolves the issue.

You can also run the Diagnose and solve problems on your function app and search for "Timer Trigger Issue Analysis" to get the insights and recommendation.

The other reason could be a restart and I will suggest you to check the Web App Restart detection section.
https://github.com/Azure/azure-functions-host/wiki/Investigating-and-reporting-issues-with-timer-triggered-functions-not-firing
https://github.com/Azure/azure-webjobs-sdk-extensions/wiki/TimerTrigger#troubleshooting