I don't understand if there is any duplication of information between the message field in the traces table in Azure Application Insights and the log records that I have in the table Function App Logs that I can export from my Azure Function Apps.
I've tried to look for it in the documentation and did some tests comparing the data but it still hasn't become clear to me.
Yes, there is duplication as both Application Insights and the export of Diagnostic settings contains the traces emitted by the function. However, Application Insights captures a lot more details like outgoing dependency calls etc. From that point of view you might consider not using the Diagnostic settings.