Seeking Advice

I read about your different topics/discussions but still have issues.
The group I work for needs to track Customs entries, compile them for payment and make reports. They used to track the entries in Excel but they wish to do this in a Form = INVOICE TRACKING, be able to query and make reports. My Access knowledge is limited but I have created two tables and the above mentioned Form

The main table= Invoice Tracking (as per the group request) as 37 fields. Starting at the 8th field, data is updated by the users on a daily basis.
The source table = XLS 1127 as 7 fields and is refreshed and updated 2-3 times weekly by a live report on the Internet.
Up to now, when I need to update the main table, I run an APPEND QUERY. Once this is done, I run a FIND DUPLICATE QUERY and I delete the duplicates manually because some of the first 7 fields will get updated this time or some other time and therefore it will create duplicates Entry Nbr in the Main table. Meanwhile the other 30 fields are populated by users with the information supplied by the invoices.
Is there a macro or an expression that could be created to avoid the Query from selecting the duplicates randomly? I have a very basic knowledge of macros and VB but would it be possible to write something that would be like In record #1, IF the field 2 or 3 is empty than look at field 9 and others and if Not Null then get the data in the duplicate record #2, merge the
information from the two records??
When the Main table will grow bigger, it will be too much time consuming to delete XXX duplicates and can cause errors.

Thanks