Revert "Replace use of dict for duplicate removal"

This reverts commit 48bcbbfc92546618d09843f38804769341eaaa61.

The changes introduced by 48bcbbf cause a "'str' object does not support
item assignment" exception; this patch restores the previous behavior
until a more permanent fix to the issue 48bcbbf attempts to address is
made.
Merge pull request #37 from ajablonski/master

Revert "Replace use of dict for duplicate removal"