When we create a new lead and qualify if the account and contact already exist for that lead fields will be automatically filled for the selected account or contact 

we can also choose whether to create contact,account or opportunity for that lead 

To Enable this feature Administration--->System Settings select option to No

Now once  Lead is qualified it will ask to create contact,account or opportunity

There is also an enhancement where you can add attachment in note will be associated to the opportunity 

This note will appear in Opportunity when qualified

This enhancement will helpful to get lead information faster