That Obscure Object of Confusion

I just watched Luis Buñuel's last film, That Obscure Object of Desire (1977). Even I couldn't help noticing that the character Conchita (the obscure object of desire) was played by two different actresses Carole Bouquet and Ángela Molina. The first time I noticed this, one actress leaves, another one returns, but all the other characters don't seem to notice the difference. What the hell was going on? Strangely enough, I quit noticing the difference too! I can recall the first scene in which I noticed the switch but after that, I didn't notice the further switching at all! In the script, there really was only one character. As an experiment, the filmmakers just chose to use two different actresses for the same part and exchanged them seemingly at random.
Even more weirdly, the two actresses don't really look much alike!