OK, it depends on a bunch of things….
If you did not choose to ignore errors on create, you will get an error message saying the table already exists and no rows will be imported.
If you chose to ignore errors and the tables are an exact match and with no primary key or unique indexes, the rows will be appended to the table. If there are unique indexes/primary keys enabled and any rows have common values, those will be rejected with ORA-000001. All other rows will be appended to the table.
If you chose to ignore errors and the existing table does not have the exact same columns and definitions, you will get an error for each row saying that the columns do not exist.
If the tables have the same column names, but the datatypes are not exact (differing lengths on varchar2 types or precision issues with number types) then import will load what rows it can and reject with an error any rows that it could not load.
Hope this helps…