How can I get a degree in one subject in the minimum possible time?

1. As you have already completed the one-degree course, it will not be difficult for you to do another degree with different subjects.
2. But there is no chance for you to take only one subject. You have to write all the subjects as required for completing a degree course.
3. You can complete your degree in one year. CIIS India is the site who are offering this course. Their contact details are
CIIS - INDIA
G-10, Sej Plaza,
Besides Nutan School,
Marve Road, Malad (W), Mumbai - 400 064 (M.S. - INDIA)
Mob: +91-9960567833
E-mail: ciismumbai@gmail.com
Website: https://www.ciisindia.in.
In addition to the above, you can also go to IGNOU local study centre and enquire. I heard there is a facility in IGNOU also for completing the course in one year.

There is a provision of doing a degree course squeezed in one year and it has to be undertaken in correspondence mode from institutes like IGNOU. This is available to all those students who had either a 3 years break after their 10+2 and partially attended the degree course or failed in it or passed it with some other subjects. Even those who did a diploma or a job in a company can go for degree after such a break of 3 years after 10+2. They have to show some documentary proof in respect of any of the above activities during that gap.
One can select one's new subjects and prepare for giving the exam at the end of one year only. I feel IGNOU is a good choice for that and one can also search any other university that is UGC-DEB approved.
Some such approved universities are Nalanda Open University, Tamil Nadu Open University, Jamia Millia Islamia, Anna University, Mangalore University, University Of Mumbai, and University of Delhi.

If you are thinking on the line of acquiring a degree with one subject only, you should contact IGNOU centre for the persuasion of the same. They entertain aspirants fit for fulfilling their wishes for studies for additional subject or subjects required by the aspirants.
Other open universities in the similar lines are as follows_
1) Nalanda open university
2) Anna University
3) Mumbai University
4) Delhi University
5) Utkal University etc