Is Bhisma repeating the same mistake

Don't worry - the Amba situation won't repeat itself since Gandhari chooses to blindfold herself after learning about Dhritarashtra's blindness. And yes, Gandhari & Dhritarashtra do get married and have 100 sons and 1 daughter.

Mahadev's boon to Gandhari of having 100 sons is the main reason why Bhishma chose to ask for Gandhari's hand for Dhritarashtra. I mean, for a royal family which, at one point, HAD to resort to niyoga in order to carry the family forward, having a daughter-in-law who becomes a mother to 100 sons would be extremely welcome.

If you think from Bhishma's perspective...he was not wrong...Everyone assumed that Dhritrashtra would be the next king...he was powerful but he needed a wife who could be his eyes. Gandhari was not only immensely beautiful, she was also learned, immensely talented but also knew how to fight. She was not the usual milk sop princess and thus a right choice for the new king of Kurus. The fact that she was blessed to mother 100 sons was an additional attraction.

Ofcourse in all this Bhishma managed to overlook that Gandhari might have some dreams of her own and Dhritrashtra might not be what she wanted but the princesses were always used as a pawn in marraiges to further alliances and gain allies. Gandhari was pawned off but to the most powerful kingdom at that point in time so she had ensured a powerful ally for Gandhar and a place in history for herself...what else could she want?

It is unfortunate that she decided to follow her husband in darkness which probably made her parents feel guilty and brother vengeful.