https://screenrant.com/full-house-olsen-twins-single-actor-mary-kate-ashley-credit/

Twins Mary-Kate and Ashley Olsen shared the role of Michelle Tanner in Full House, a practical solution for the show that came with a risk.

If you listen to the commentary track on the Season 1 DVD of Full House the commentary mentions that when Mary Kate And Ashley Olsen started playing the role of Michelle when they were babies the producers and writers of the episodes didn't credit them at first since they were babies but as they grew older they finally got their credit as Mary Kate Ashley Olsen without the "and" added. They also did the same with the twins who played Jesse and Becky's twins Nicky and Alex when they were added to the cast later in the show
Bestie

I love how the producers decided to show that there was more than one girl by having one play Jesse's Greek family member in the season four opener. :) :) :)

God bless you and the girls always!!!

Holly (a day one fan of them)