I've seen people often use "recurring again".
Recurring means "occurring again"; then why do we have to add "again" along with "recurring".
Which one the following is correct way to say it ?
The sense of recur you have in mind does indeed mean ‘to occur again especially periodically or repeatedly’ (OED). Those of a logical turn of mind will say that recur again, if it means anything at all, can only mean that something occurs again again, but I don’t suppose those who say it mean any such thing. The collocation is certainly found, with seven instances in the Corpus of Contemporary American English and two in the British National Corpus, but those who wish to avoid censure will do well to avoid it. Recur is enough.
Saying recurring again is "okay," yes, in that it is grammatical and comprehensible, but it's virtually always unnecessary and generally inadvisable.
As OP notes, the concept of repetition is included within the word itself, and thus adding the word again is redundant and may potentially annoy people.
There may be times, however, when one wishes to emphasize that something is once again occurring again, in which case "recurring again" is the logical way to express the idea.
Aside from that particular type of situation, stick with recurring.
It's quite like revert back, if you ask me. For emphasis or any other reason, it is better to rephrase than use a redundancy such as this.
As for the corpora, I am not sure if the collocation was a poetic usage or similar, in which case, strict grammar may not be applicable.
Recurring itself means occurring again so recurring again is incorrect in my opinion
In my view : "Confirm that it will not happen again." is better than your options.
Oftentimes, it is not clear whether the meaning of recur is to imply a one-time occurrence or multiple occurrences, in which case one can use occurring persistently rather than recurring again.
