What is the difference between 碰，触，碰触 and 触碰
觸 is very rarely used alone in modern Chinese. 碰 however, is, and in fact frequently so. You see it in 別碰！ (Do not touch!), for example.

觸碰 is far more common than 碰觸 albeit meaning exactly the same.

觸碰 is more written than 碰. So you see signs written like 請勿觸碰 ('Please do not touch'), but never *請勿碰.

碰 is also polysemous, so if you intend the meaning 'to come across sb.', as in 我碰到他 (where I intend to mean 'I came across him' here; it also can mean 'I touched him'), you should not write it as 我觸碰到他, which can only literally mean 'I touched him' (also more written).