It is legal in Islam for a man to marry more than one woman.
فانكحوا ما طاب لكم من النساء مثنى وثلاث ورباع
Then marry those that please you of [other] women, two or three or four.
— Quran 4:3
If the marriage was proper, e.g. in the presence of two witnesses and the wali of the bride, with proper offer and acceptance etc. then it is a valid ...