The exact consequence of filling two bubbles for a single question in CUET depends on the official marking scheme, which may not be publicly available. However, it is generally not recommended and can lead to negative marking.

This is the most likely scenario. Filling two bubbles might be considered as marking both options as wrong, leading to negative marks for that question.
In some cases, the question might be considered unattempted if it's unclear which option you intended to choose. This would mean you wouldn't receive any marks (positive or negative) for that question.

https://university.careers360.com/download/ebooks/all-about-cuet

I hope it helps !