I am trying to generate unique token id, can i use mt_rand()?
will mt_rand() generate same number twice?
mt_rand will generate the same number twice, yes. Every random number generator will probably do so eventually. (Theoretically) every number has the same chance of being generated every time the generator is run. It could randomly generate the same number many times in a row. It's random.
To use a random number generator for unique ids, the probability of generating the same number twice must be so low as to be irrelevant in practice. In this regard, mt_rand is probably not sufficient. The concept of randomly generated unique ids has been formalised into UUIDs, which you should use for exactly the purpose of a universally unique id.
Take this quote:
...only after generating 1 billion UUIDs every second for the next 100 years, the probability of creating just one duplicate would be about 50%.
Since mt_rand returns a 32-bit integer on 32-bit systems, it can only return 2^32 unique values, which is a mere 4,294,967,296 unique values. If you were generating a billion mt_rand values every second, you're basically guaranteed a duplicate after 4 seconds. That hopefully illustrates the difference of scale between UUIDs and mt_rand and why that matters. Even if you're generating much fewer than 1 billion ids every second, you'll still need to choose an algorithm which makes collisions practically impossible, not just unlikely.