Here is the answer that I obtained after that I have written to the technical support :
" Currently it is not possible to merge duplicates. You will need to remove
the double entries manually. To do this, please select the card of the
individual and go to the Person menu > Person > Delete Person. "
I have found the ideal solution and it is free, When uploading any files you allways have the potential duplicate problems The solution is to go on to the Mormon Church Family History Website and download their Geaniology software prograhm. This prograhm has the facilities to tell you what duplicates are in your data and allows you to correct the problem. Set up a file and export your gedcom data to this file. Then download it to your new Prograhm that you have just downloaded. When you have completed the merging of duplicates just do the transfer in reverse and transfer the sorted data back new updated family tree site into your Hertitage or other soltware prograhm. I do this excercise regularly as it keeps your research nice and clean.
This is much easier to clear up your duplicates than trying to do it manually. The problem with doing it manually is that it is impossible to find all your duplicates especially if you have a large tree.