The Web to Lead app gives you a web form to collect information from potential customers (or leads). A lead is a potential customer. Therefore, leads are not synced to QuickBooks until the lead is converted to a customer.
When the Web to Lead form is filled in, it does a few checks:
The Name given to the Customer Lead will be the entered Company Name. If the Company Name field is left blank, then the Name used will be the first and last name.
Method:CRM searches to see if this Name matches other names in your customer list. A match is found if a record starts with the entirety of the name. Thus, if the entered name was "Pop and Skate" and Method:CRM has an existing customer called "Pop and Skate Inc" then this is considered a match.
Method:CRM searches to see if the entered Email exists in Method:CRM.
Based on what it finds, the Web to Lead form continues as follows:
If a Name match is not found, a new lead is created.
If a Name match is found and the email also exists, then no new lead is created.
If a Name match is found but the email doesn't exist, then a new lead is created with the Name followed by a number indicating the number of existing matches. If that new name also exists, then a Name is created appended with the current date.
In all cases, a new activity and opportunity will be created.
The above three cases cannot account for every situation where a customer already exists in Method:CRM. If you find a duplicate has been entered and they have converted to a customer and thus synced to QuickBooks, you can manually merge those two records together in QuickBooks.