Can Weaponmaster take an already equipped equipment from another unit and equip it on its own?

Yes, a unit with the Weaponmaster keyword can take an equipment that is already attached to another unit and equip it to itself.

According to the FAQ and Rule 747.1.b, Weaponmaster allows you to choose an equipment you control and attach it to the unit with the Weaponmaster ability, even if that equipment is already attached to a different unit. This effect is voluntary.