Difficulties are that

1. you also want to avoid duplicating dentistry research outside India.

2. titles for the same work in India may have different titles.

3. results are reported not just in theses but also in papers and books.

4. real research is multidisciplinary, and you will miss important methods that are not labelled "Dentistry".

5. real research requires you to collect your own data, not rehash someone else's data.

Positives are that you can get quick initial access to solved local problems and their local solutions.

Summary -5+1= -4

Sir I really appreciate your response and am in sync with the validity of your observations and have following are my interpretations 

• difficulty 1. Maintaining a central repository for thesis can actually prevent duplication of work not in India but outside India
• Difficulty 2. In case same work is done under different titles, the principal investigators can be contacted regarding similarity of work, again maintaining a central registration can help
• Difficulty 3. This particular question pertained to thesis performed in different institutions as many institutions might be working on similar projects not knowing about each other's work so a central registration would help. However you are completely right that results are  published in books and articles but that would be considered in this registry if they are  publications from thesis.
• Difficulty 4. You have raised a very pertinent point that real research is multidisciplinary and in agreement, thesis here too are many times conducted in integration with multiple disciplines  of medicine and even engineering or genetics etc, so you are  right instead of naming it under dentistry it should be some integrated post graduate thesis registry. Thanks a ton for that observation.
• difficulty 5. True that everyone should collect their own data but in case of central registration and standardising of criteria for similar studies, data can be pooled

This is in fact a very good idea. 

Hats off to that.

With some good efforts and dedicated team work, the universities and authorities can very well do this.

The planning, organisation and execution will take some time.. but it's certainly achievable. 

And once done it will be very useful and worthy of the effort. 

Dr Bronson

I thank you for your inputs and recommendation, I sincerely wish by this question I am able to generate ideas from  researchers worldwide what can be ethical and organizational problems in initiating this repository , if maintained for one particular country or for all ( like sites for systematic reviews, mandatory registration in PROSPERO) , one problem I feel can ideas sharing which may not be liked by all thesis guides. 

Getting a central repository is possible. The universities where the thesis is registered and being conducted should send the list (title and abstract) to the UGC. The UGC should be maintain this information and display it on their website for everybody's reference. The complete draft of the project can be requested from the authors if need be.

Dr Anand....I really appreciate your feedback.. you are right we can maintain a national database in UGC ore any other governmental body we people decide.. but If we all are in sync we can formulate a regulatory body we can  have a vision of going many steps further... based on your experience sir, it would really be beneficial if you can contribute some of your ideas that can make thesis in our country more standardised giving possibility of pooling data  in different streams of research .

В Украине есть центральное хранилище диссертаций (полных текстов) и авторефератов с 1994 г. 

http://www.uintei.kiev.ua/viewpage.php?page_id=57

По всем отраслям знаний, включая медицину.