You can use VLookup on the new names against the old list and if the name is
found do not add it.

Example:
Cell A2 Cell B2
New Name1 =VLookup(a2,Range of main list,Column Offset,0)

If it returns an #N/A then this is a new name. If the name exists then the
formula will return the name.

If you need to test two cells at the same time like Last Name First Name
where they are not together in the same cell then you may have to concatenate
the name for example: Column A has Last Name and Column B has FirstName
then you can put in Column C =A2&" " &B2 and then do the same with the new
data and then do the lookup.

Hope this addresses your question correctly.

Steven