You try to email overdue invoices and multiple AvaTax windows appear.
When a user emails any document, QuickBooks generates modify event for that document and AvaTax captures this event and calculate tax accordingly. QuickBooks SDK do not provide any mechanism to distinguish between email and modify event. The same modify event will be received if a user switches from one template to other and hits the "Save" button.
Unfortunately, we cannot fix this issue because of QuickBooks SDK's limitation. Our engineers raised ticket for this to Intuit and they have replied stating there is no way to distinguish between both the events.
As a workaround you can turn off the AvaTax integration for the desired document type from "File -> AvaTax -> Configuration -> Tax Calculation tab -> Disable automatic calculation on Sales Document" option.