Is it possible for someone who was not a Jew or Christian, if he becomes Muslim, to attain multiple rewards as the former Jew or Christian will attain a twofold reward?
Question

There is someone who embraced Islam and was Christian before. As for me, I was an atheist. Will those people attain the twofold reward because they were previously People of the Book? I ask Allah to bless them. All it is, is that I feel sad because I had to go through a lot of anguish before I came to know Allah, may He be blessed and exalted.
Praise be to Allah.Al-Bukhari (3011) and Muslim (154) narrated from Abu Musa that the Prophet (blessings and peace of Allah be upon him) said: “There are three who will be given a double reward: a man who has a slave woman whom he taught and taught her well, and he taught her manners and taught her well, then he set her free and married her – he will have a double reward; a believer from among the People of the Book, he was a believer, then he believed in the Prophet (blessings and peace of Allah be upon him) – he will have a double reward; and a slave who fulfils his duty towards Allah and is sincere towards his master.”
Whoever believes in his Prophet then believes in Muhammad (blessings and peace of Allah be upon him) will have a twofold reward, one for believing in his Prophet (peace be upon him) and one for believing in our Prophet Muhammad (blessings and peace of Allah be upon him). This is in accordance with the words of Allah (interpretation of the meaning):
{Those to whom We gave the scripture before are the ones who believe in it [the Quran].
When it is recited to them, they say: We believe in it, for it is indeed the truth from our Lord. Verily, we had already submitted [to Allah, in Islam] before that.
Such people will be given a twofold reward for their patience and steadfastness; they repel evil with good, and spend out of that which We have provided for them} [al-Qasas 28:52-54].
This is specifically for those of the People of the Book hwhoe believe in him, so it does not include all disbelievers. al-Hafiz Ibn Hajar (may Allah have mercy on him) said:
The hadith applies only to the People of the Book, and it does not include others. Moreover, what we understand from his saying “he believed in his Prophet” is to highlight the great reward; in other words, the reason for the twofold reward is believing in two prophets, and the disbelievers are not like that. It may be said that the difference between the People of the Book and other disbelievers is that the People of the Book recognize Muhammad (blessings and peace of Allah be upon him), as Allah, may He be exalted, says (interpretation of the meaning):
{those who follow the Messenger, the unlettered Prophet, whom they find mentioned in their own [scriptures], in the Torah and the Gospel} [al-A‘raf 7:157].
So whoever among them believes in him and follows him will have an advantage over others."(Al-Fath  1/191).
That is supported by the report narrated by Imam Ahmad (22234) from Abu Umamah, who said: I was beside the mount of the Messenger of Allah (blessings and peace of Allah be upon him) on the day of the conquest [of Makkah]. He spoke some pleasing words, and among the things he said was: “Whoever among the People of the two Books becomes Muslim will have a twofold reward, and his rights and duties are the same as ours. Whoever among the polytheists becomes Muslim will have his reward, and his rights and duties are the same as ours.”
Classed as hasan by al-Albani in al-Sahihah (304).
Thus he differentiated between those of the People of the Book who become Muslim, as he said that they will have a twofold reward, and those of the polytheists who become Muslim, as he did not say that they would have that.
The fact that those of the People of the Book who become Muslim will have a twofold reward does not mean that they are better than others in definitive terms. Someone else, who was a polytheist or disbeliever and not one of the People of the Book, may become Muslim and practice Islam well, so he will be given a reward many times greater than that given to one of the People of the Book who becomes Muslim, because of his sound adherence to Islam, the strength of his faith and his righteous deeds. How many of the disbelievers became Muslim and Allah granted victory at their hands and benefitted the people through them, and many people became Muslim at their hands and many nations entered the religion of Islam because of them, such as Abu Bakr and ‘Umar (may Allah be pleased with them). They are better and attained greater reward than Salman al-Farisi, ‘Abdullah ibn Salaam and others like them who were previously among the People of the Book –may Allah be pleased with them all.
Al-Hafiz Ibn Hajar (may Allah have mercy on him) said:
Not everyone who receives a twofold reward is better than others."(Fath al-Bari by Ibn Rajab 2/232).
You have hope, depending on what you do after you become Muslim. If you adhere sincerely to Islam, strive to do righteous deeds and remember that Allah is watching you and what you say and do in private and in public, and you devote yourself to your religion, learn its rulings and teachings, memorize His book and teach that to others, then you will attain of reward what no one knows except Allah, and perhaps you will have many times more reward than someone who attained a twofold reward.
If you suffered and went through anguish before, because of disbelief and being with disbelieving people, until Allah blessed you with Islam, then you should acknowledge the blessing that Allah has bestowed on you, give thanks to Him, may He be glorified, for His immense grace, and focus on worshipping and obeying Him. Do not think that what you missed out on of reward means that you cannot attain a better, greater and more generous reward than that. In fact, it is possible to attain that by the grace of Allah, so do not think too much about what you have missed out on of reward, and focus on what you should not miss out on, by doing righteous deeds and fearing Allah.
And Allah knows best.