A friend of mine asked me this:
A person said his first Shahada and became a Muslim, but not as a follower of the Shia school of thought. Later on, he became a follower of the school of Ahlul-Bayt and said the Shahada again (basically a second shahada).
In all these years, since the first shahada, the person did not pray and he also didn't start praying after becoming a Shia. Now, after some time, he wants to start praying. Does he have to repeat the prayers that he missed since the first Shahada or does he "only" have to repeat the prayers that he missed since becoming a Shia/saying the second Shahada?

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Yes he has to make up all those missed prayers since he became Muslim. When one becomes Muslims, he is obligated to observe the prayers.