Open the profile in EDIT mode. Look on the right side, below all the listings of family members. Find
[duplicates? please merge]
Click on it, You will find all the children, siblings, spouses, and all of them will have little sqare grey boxes on their left. When you find two alike, click first on the higher number, then on the lower number.
Off on the right you will find a yellow box with 4 options. I always use the "Compare" option, This allows me to verify that there is truly only one individual represented. Once you have done this, click on Merge at the top. If you are on the trusted list for both profiles you can then merge them, if not, explain in the box presented why you think they should be merged and complete the requirements there.
This may have changed since I used it, things seem to be changing almost daily here.