Add option (on by default) to remove duplicates

[dakotahawkins]

While sorting, removes duplicate include directives

Notes:

• Only works within batches
• Should respect $include-toolbox-preserve$
• Case sensitive check against include content + delimiters

[dakotahawkins]

This was a frequent annoyance in files with very long lists of includes. I was using IncludeToolbox and then checking manually (when I remembered), but that's not foolproof :)