Hi @globert / Thiago
The HubSpot count of form submissions is more accurate in this scenario because this is counted on actual form submissions received (contacts created) regardless of any potential tracking issues. For example if a user has a cookie blocker extension in their browser then GA would not be able to count the submission, whereas HubSpot would.
In the example of the cookie blocker HubSpot would lose attribution but it would still have the actual form submission. For this reason I prefer to build out all my reporting in HubSpot directly. GA / GA4 is perhaps good for comparison but a question could be whether you really need to build your reporting in GA if you already have HubSpot?
With regards to GA4 duplicating the submission count - this could potentially be down to GA4 automatically picking up form submissions and counting these as a submission. e.g. I’m not sure that you need the HS form listener in GA4 and so perhaps it is seeing this conversion twice - once natively and once due to form listener.
Not sure if that helps and good luck with getting to the bottom of it.