Hi there! Google's stance on duplicate content is that it will only be penalized if Google thinks you're trying to scam people. So no, you won't be penalized for having the same descriptions for your Squarespace videos as you do for YouTube.
In Google's words, "Duplicate content on a site is not grounds for action on that site unless it appears that the intent of the duplicate content is to be deceptive and manipulate search engine results."
You can check out this article for more: https://developers.google.com/search/blog/2008/09/demystifying-duplicate-content-penalty
EllaSEO's post in Potential Duplicate Content With Embedded Video page & Youtube was marked as the answer
