You try to email invoices or 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, Avalara cannot fix this issue because of QuickBooks SDK's limitation. Avalara raised a ticket with Intuit regarding this matter. At this time, Intuit does not have the ability to distinguish between both the events (email and modify).
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.