Look at the mothers profile = Duplicate mothers - with different profiles, will show a half sibling.
The half sibling is very useful because it indicates that something is wrong if you know different - a sort of little error report all on its own
First the mothers has to be merged and then after that the duplicate sons must be merged and all will be well again.
Just remember to first do the mothers
Please ask if you need help. I am just an e-mail away.