Add onDuplicate callback

[jfposton]

Having an onDuplicate callback would be useful for authors who wanted to have duplicates and/or those who wanted to do some custom action or extra check when a duplicate item is introduced.

The onDuplicate callback would either return true or false. If true the duplicate is added if false the tag is not added.

The onDuplicate callback would take two parameters, the original tag and the value that would be added if the callback returns true.