Finding/Marking Duplicate Values using SQL - sql

Question

I am trying to update a column in my table to specify if the record is a duplicate or not. To do this I added a field called 'DuplicateRecord'.
I can't use the query wizard in access as the duplicate option in this only allows for 10 fields to be checked as a duplicate and my table has more than 10 fields.
The below code works for me:
Call Module1.RunSQL("UPDATE myTable " & _
"SET myTable.DuplicateRecord = TRUE " & _
"WHERE myTable.[CompanyID] IN (" & _
"SELECT * FROM " & _
"(SELECT myTable.[CompanyID] " & _
"FROM myTable " & _
"GROUP BY myTable.[CompanyID] " & _
"HAVING COUNT(*) > 1 ) T1 ) ")
However this code just runs off one field and I need it to run off all fields in the table (there are about 15 fields).
As a test I tried using two fields to see if I could get this working using the following:
Call Module1.RunSQL("UPDATE myTable " & _
"SET myTable.DuplicateRecord = TRUE " & _
"WHERE myTable.[CompanyID] AND myTable.[Product] IN (" & _
"SELECT * FROM " & _
"(SELECT myTable.[CompanyID], myTable.[Product], COUNT(*) " & _
"FROM myTable " & _
"GROUP BY myTable.[CompanyID], myTable.[Product] " & _
"HAVING COUNT(*) > 1 ) T1 ) ")
However I get an error message saying "Run-time error '3306' You have written a subquery that can return more than one field without using the EXISTS reserved word in the main query's FROM clause. Revise the SELECT statement of the subquery to request only one field."
I've tried googling the error but I can't seem to solve it as I don't fully understand it.
Does anyone know how I can apply the logic of testing for duplicates? Is there an easier way to do this then how I am currently trying? I will need to do this for the full record (15 fields), so I am a bit conscious that the way I am currently attempting this might not be the best fit.