The Latin hodie, today, was an amalgamation of hoc (this) and die (day). The Italian oggi, the Spanish hoy, the Catalan avui and the Portuguese hoje are all direct descendants of hodie, via Vulgar Latin.
The French aujourd’hui has a slightly more convoluted relation to its semantic ancestor. The particle ‘hui’ is derived from hodie, whilst ‘aujourd’ is a conglomeration of au jour de, meaning ‘on the day of’. If we unpack the word aujourd’hui we therefore end up with ‘on the day of this day’ (the name British news spoof The Day Today made use of a similar redundancy).
Aujourd’hui has been in use since the 16th Century, and endured long after one of its components – hui – ceased to have an independent existence. We could speculate that as hui was lost, the pleonasmic nature of aujourd’hui became less evident.
Since the 1700s, the phrase ‘au jour d’aujourd’hui’ has been in use in French: this a doubly redundant formulation, which could be rendered into English as ‘on the day of the day of this day’, and is used in the sense of ‘nowadays’ (the Spanish hoy en dia has a similar sense).