I did a 'parent search' on familysearch.org for Alex's parents, Charles and Jessie (nee RITCHIE) and up he popped! While checking the information, I noticed that his birth date was identical to brother William's but perhaps this was confused with a christening date (children were sometimes baptised in 'bunches').
I managed to download the birth entry on Scotlands People and sure enough, John and William were twins!
Birth record of Charles & William BUCHAN, Peterhad 1868
Anyone else found twins in their families? I think this was the first instance I've come across.