Why is it not possible to specify the same field as specialty, working and interest field?

Once a field has been added to an area (as specialization, working or interest field) through the Profile updater, it will no longer be available in the list of fields to be added to one of the other two areas.

The logic behind this is that if you report a field as a specialty field, then you work and you are interested in this field. So, no need to add it in all three areas. In the same way, if you work in a field then it means you are interested in that field but you are not necessarily an specialist. Finally, being interested in a field does not mean you are an specialist or that you work in that field.