He did ‘umrah believing that ‘umrah is Sunnah, then he realized that the view that it is obligatory is the correct view. Should he repeat it?

Firstly:

We have previously mentioned the difference of scholarly opinion regarding ‘umrah in the answer to question no. 39524 , where we noted that the view favoured on our website is that it is obligatory once in a lifetime, like hajj.

Secondly:

If someone did ‘umrah after reaching the age of puberty, in the correct manner, that is sufficient for him and he has done what is required of him, even if he believed that it is Sunnah and is not obligatory. That is because ‘umrah does not need to be accompanied by the belief that it is obligatory when intending to do it, or to specify whether one is doing ‘umrah as obligatory or Sunnah. Rather it is sufficient to intend to begin the rituals [by entering ihram].

Al-Mawardi said in al-Hawi al-Kabir (2/92): All acts of worship are of three types:

One type which requires the intention to do the action and nothing else; another type which requires the intention to do the action and believe that it is obligatory and nothing else; and a third type which requires the intention to do the action, belief that it is obligatory and to specify what actually is to be done.

As for that which requires the intention to do the action, without believing that it is obligatory or specifying what is actually to be done, this applies to hajj, ‘umrah and purification. So when a person intends to do hajj or to do ‘umrah or to purify himself for prayer, that is valid, even if he does not intend that it is obligatory and specify what he is actually going to do… End quote.

And Allah knows best.