I need to merge two IDs of duplicate person which are shown as mother and daughter

A parent can't be merged with their child. So you will need to go back to the profile page and remove the parent-child relationship. Then you will be able to merge the two profiles. This article can teach you how to remove that relationship: https://www.familysearch.org/en/help/helpcenter/article/how-do-i-correct-parent-child-relationships-in-family-tree