This error means you have the same parameter name repeated, giving duplicates in the final URL.
The final URL is generated from the base URL + the parameters in the data passing section.
The most common issue is having e.g. https://domain.com/?subid=xxx in the base URL then having the field "subid" defined again in data passing.
So, as a rule of thumb, pass everything dynamic using the data passing section (i.e. where you're passing FunnelFlux data like hit IDs, funnel IDs, etc.), and in the base URL just have the minimum functional link that would go to the desired destination, such as your lander URL directly, or if its an affiliate link, something like https://affnetwork.com/?affid=xxx&offer=yyy -- here you would have things like affiliate and offer ID in the base URL as they would be different for each offer, so aren't something you can template easily in the data passing section.