Geoff
I don't know about the regimental numbering system pre 1881 but certainly from 1881 once the number had been issued to you, it was yours and remained with you for as long as you were a soldier (unless of course you were commissioned). As a Private soldier or NCO, the number remained the same regardless of rank and once issued it was not to be re-issued (so if you were unlucky enough to be killed in action or die during service, that number was not to be re-issued).
However, and again I'm talking about post 1881, numbers were allocated within certain ranges and once a regiment approached the end of a particular series, application had to "be made to the Adjutant-General in sufficient time to obtain authority to commence a new series."
Up until 1904, that limit, for infantry of the line, was 9999. Once that figure was reached the numbering commenced from 1 again. All this is a roundabout way of second guessing why you
could see two identical numbers in the same regiment.
I've covered this topic (ie numbering series post 1881) in a couple of posts which may be of interest:
Regimental numbering seriesThe Queen's & King's Regulations - Regimental NumbersAliases were sometimes used at the time of enlistment but your man wouldn't have been allowed to change his name mid-way through his service. I think you're looking at two different records here.
Paul