The way I do it is in the Query your using to append your records add the table your appending to. Use a left outer join. Look at the sample
Code:
Insert Into table1
Select table2.ProjectID, table2.Error,Now() as [Date identified]
From table2 left outer join table1 on table2.projectID = table1.projectid and table2.error = table1.error
Where table1.projectid is null
table1 in the sample above is the data you're appending. table2 is the table your appending too.
the left outer join is linked on both the projectID field and the error field because you can have multiple errors with the same projectID so you must check the error as well.