Is there any problem with praying the same salah twice?

Question

I have 2 cases two discuss.

1. I offered my salah well within time at my home, alone. I walked out of my house and saw a few of my friends who were going to the masjid to pray the same salah as a group (jamaat). I joined them and prayed the same salah again as a member of the group.
2. I offered my salah. But my mind was not in it. I am not sure if I prayed 3 or 4 rakahs. Not sure if I did the two sujoods or not. Feeling discontent, I offered it again.

Are these actions fine within Islam? Is it makrooh/mushtabbh to do so? Please answer with evidences/references.

Answer 1

Case number 1 seems fine according to this Hadith

Jabir bin Yazid bin Al-Aswad Al Amir told us that his father said: "I attended Fajr prayer with the Messenger of Allah (ﷺ)in Masjid Al Khaif. When he finished praying, he saw two men at the back of the people who had not prayed with him. He said: 'Bring them here.' So they were brought to him, trembling. He said: 'What kept you from praying with us? They said: '0 Messenger of Allah (ﷺ) we has already prayed in our lodgings.' He said: 'Do not do that. If you have already prayed in your lodgings, then you come to a Masjid in which there is a congregation, then pray with them, and it will be a voluntary prayer for you."'

Grade   : Sahih (Darussalam)    
Reference: Sunan an-Nasa'i 858
In-book reference: Book 10, Hadith 82
English translation: Vol. 1, Book 10, Hadith 859

Case # 2: If you forget Salah you should do Sajada Sahaw

Reference when you forget something http://sunnah.com/abudawud/2/619

Don't feel discontent when you are performing things according to Sahih Hadiths