If one had children from his late brother’s wife, fulfilling the Mitzvah of Yibbum, did he also fulfill the Mitzvah of being fruitful, or do we say that for this purpose they are not considered his children, but his late brother’s?
The פסוק's words וְהָיָה הַבְּכוֹר אֲשֶׁר תֵּלֵד יָקוּם עַל־שֵׁם אָחִיו הַמֵּת (דברים כה-ו) is explained by the אבן עזרא that the firstborn of the יבום should be named after the deceased brother. The רמב"ן proves from בועז that there is no need to do so and says that על פי קבלה the פסוק is telling us that the baby will be a גלגול of the deceased brother. רש"י learns that the meaning is that the one who does יבום gets the inheritance in the father's estate that the deceased would have received. According to none of these ראשונים is there any indication that the child is considered to be the deceased child, and as such there is no reason to assume that the מייבם shouldn't have fulfilled פריה ורביה through this child. However, according to the ספורנו the פסוק tells us that through the child, the deceased brother will have fulfilled פריה ורביה. Your question would indeed be a question according to the ספורנו's interpretation.