Hi Ronnie
And thanks for posting a question to the helpline.
The back in these examples is indeed redundant: he returned... and Please revert.. are clear and correct without the need for back.
Redundancy - including words or letters that add nothing to the meaning or form, sometimes by simple repetition - is an interesting thing! Some examples have become fixed expressions, such as free of charge and personal opinion. A term like close proximity is so well established that we hardly notice that in a sentence such as I live in close proximity to the school would work just as well without close.
Regards
Workplace English Expert