What is the concept of "Wahdat al-Mawjud" in Shia Islam and is it kufr to believe in? I’m really confused about this subject. Some Shias in my community believe in it while other shias say it’s kufr which negates the oneness of Allah (swt) and end up takfiring the others.

We must know what he means by the unity of existence, in order to be able to judge.
If he means that the real being is one and that is Allah Almighty, and the rest of the existing beings are imaginary or shadow beings, then this can be accepted to some extent, and it is not blasphemy.
But if he means the unity of existence and the actual existence without any plurality and plurality in between, so the existence is one and the existing is one, and accordingly, Allah and the creatures are one thing, but the difference is by consideration. Hence, this one existence is in the Creator a creator, and it self is in the created creature, as in the sky it is sky, and on the earth its is earth, and so on.
This is totally and completely rejected, and it is blasphemy, because we see a real dualism between the Creator and the creature, and that the distinction of the Creator over the creature is not based on consideration or observation, but rather it is a difference and a real distinction