Export and Import losing first field..

[comment]

you mention that duplicate records would be skipped. However, part of the import is to delete all the existing records first. Therefore all of the records will be new.

True, but if the source file contains duplicates, only the first one of each group will be imported.

BTW, after you recover the file, don't use your existing script to test the import, because the import map is very likely to be seriously mangled.