What do you mean by single citizenship?
Singl citizenship says that one cannot be citizens of the state as well. This helps in increasing the feeling of nationality and encourages patriotism as it forges unity amidst regional and cultural differences. It also encourages fundamental rights such as the freedom of movement and residence in any part of the nation. In India, the concept of single citizenship is adopted from the British constitution that is from united kingdom. In India, only single citizenship is available to citizens.
In a federation there is basically double citizenship. A citizen belongs to the State where he is born and also enjoys the citizenship rights of the Federation, to which his state has joined as a unit. This is on the basic fact that the states in a federation are of course units, but do not at the same time, give up their individual entity. But in India, there is single citizenship. The citizens of the country belong to the Indian Union and not to any respective state.