What are the detailed opinions of scholars and madhhabs regarding the issue of omitting the shaddah in Al-Fatihah, and does that invalidate the prayer?

Hanafi school of thought: The prayer is not invalidated by omitting a tashdid (doubling of a letter) if the meaning does not change. If the meaning changes, there is a difference of opinion, but the preferred view is that repetition (of the prayer) is not required.

Maliki school of thought: They emphasized the obligation of preserving the tashdidat (doubled letters) in Al-Fatiha. Whoever does not master them, their prayer is invalid. As for mistakes ( lahn) in recitation: if done intentionally, the prayer is invalidated; if done out of forgetfulness or inability, it is valid; and if done out of ignorance that can be rectified through learning, there is a difference of opinion.

Shafi'i and Hanbali schools of thought: Whoever omits a tashdid in a word of Al-Fatiha, their recitation is not considered valid and must be repeated, because a mushaddad (doubled) letter stands in place of two letters, and omitting the tashdid is like omitting a letter. This is unless the person is unable.