Another book states:

Q: Salaah-91: The Imaam forgetfully omitted a Waajib act in salaah. forgot to perform the Sajdah Sahw and then made Salaam. A person who was not part of the first Jamaa'ah then joined the Jamaa'ah. Will his salaah be valid?

Answer: According to the famous book of jurisprudence Durrul Mukhtaar, the salaah of the newcomer is valid.

Although the questions are the same, the replies are different. Which is correct?

Answer: With the grace of Allaah, the reply cited in Fataawaa Raheemiyyah is correct and corresponds with the preferred opinion. The salaah performed first cannot be said to be wasted because all the postures were competed and it suffices to absolve a person of the obligation to perform the salaah. The command to repeat the salaah is merely to complete the Waajib act that was omitted so that the first salaah can be complete. Just as Sajdah Sahw compensates for an error, so too does this repeated salaah compensate for the error in the salaah. The second salaah is therefore not a fully-fledged salaah. The newcomer will therefore still have the obligation of performing the salaah because his salaah had not been performed.

Of course, if any Fardh (obligatory) posture of salaah was omitted, the entire salaah will be nullified and even Sajdah Sahw will not be able to compensate for it. It will have to repeated as a separate and fully-fledged salaah and will therefore suffice as a Fardh (obligatory) salaah for a newcomer. And Allaah knows best what is most correct.

Fatawa Rahimiyyah vol.2