The Hindi word for tea (the drink) is chai. In India, this is the primary kind of tea (also, Hindi isn’t spoken throughout India); so, IndE speakers say “I’ll have some tea” rather than “I'll have some chai”.
The drink has become popular in the West, and is referred to as chai to disambiguate it from tea prepared any other way.
However, I’ve been hearing the phrase chai tea increasingly often. Isn’t it redundant? To me, it’s like saying cappuccino coffee.
This ngram shows that it’s a recent phrase, and is growing in usage.
Hence my question: is it correct to say chai tea in BrE/AmE, or should we simply say chai?
EDIT: I don't want the answer to be opinion-based. I'm looking for a source which explains why this usage is right or wrong.