Importing duplicate values into primary key results in non-meaningful error

When you import data into a table that already exists and has a primary key assigned, if the delimited file contains duplicate values in the primary key field, the error message just says something like, "C:\xampp\htdocs\drupal\sites\all\modules\tw\tw_import\tw_import_delimited.module at line 247", instead of something helpful like, "Duplicate value in primary key" plus the troublesome line of data. This makes it difficult to find out where the problem is originating.