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* Fix for duplicate images being copied to an XlsxWriter file. Excel uses an
optimization where it only stores one copy of a repeated/duplicate image in
a workbook. XlsxWriter didn't do this which meant that the file size would
increase when then was a large number of repeated images. This release fixes
that issue and replicates Excel's behavior. :issue:`615`.
* Added documentation on :ref:`num_format_categories` and
:ref:`num_format_locale`.
* Added note to :func:`protect()` about how it is possible to encrypt an
XlsxWriter file using a third party, cross platform, open source tool called
`msoffice-crypt <https://github.com/herumi/msoffice>`_.