Make it optional for same footnote content to get same footnote number

[opoudjis]

The same footnote content may still properly get different footnote numbers in different places, which is in fact the built-in behaviour of Word. This should at least be an option.

Fn. 29 in NIST SP 800 53r4 is meant to be such repeating content, but is not rendering properly. Need to confirm that assigning the same footnote number to repeated instances of the same content is in fact still working in Word output.

[opoudjis]

I'll go further: it is undesirable behaviour in Word, since it contradicts Word expectations; and Word is where it plays up. HTML does not need to change. That means inconsistency of numbering between HTML and DOC; I don't think that's a serious concern.