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Our best guess is that the following is occurring:
The user submits a form on the site using a mobile device.
The user then opens a new browsing window, leaving the previous window open.
Time passes and the user navigates back to the page when the form was submitted.
For example, it is quite common for us to see this behavior where the user agent for the device is identical but the IP address and ISP change. In these cases, I suspect it is either someone reopening the browser at a new location triggering a fresh of all pages or maybe it is simply reconnecting to a new internet connection that triggers the refresh.
The ISP changing strongly indicate a mobile devices are the most commonly affected systems. Desktops would be unlikely to change ISPs with such frequency. Mobile devices, however, are likely to use home, office, mobile provider, and other wifi hotspots so changing ISPs many times in a single day is normal for those types of systems.
I found this Netlify Forum post that may help with eliminating the multiple form submissions: