The dual degree of IIM Calcutta provided by CEMS is of little use unless you are planning to settle in Europe in the long run. The reasons why it is not worthy are:
CEMS is not recognized in India. No industry recognises this degree. 95% of IIM graduates are placed within India.
The international scope of this degree is limited to Europe only. 21 out of 30 CEMS member schools are European.
This degree requires you to complete a 10-week foreign internship which is very difficult to secure.
The selections for CEMS are competitive. 25-30 students are selected from among 70-80 applicants.
Without any doubt CEMS is the highest rated dual degree program in Europe. But within India or outside Europe, this program becomes useless.