There's no rule against using the same auxiliary verb, including will, more than once in a sentence.
Consider these two examples, written by native speakers, found in a search for "will decide that he will":
My husband, who is a pediatrician, still holds on to a hope that Adrian, one of our youngest, will decide that he will do it after all.
(source)
and
It may well be that the circumstances are such that the franchisor will decide that he will terminate the contract...
(source)
In fact, you could have used it three times by using will before come, but it's probably better to avoid that much repetition.
So there is nothing wrong with your sentence grammatically.
You could use the simple present instead of the first future will, as in
Shyam decides that he will work hard for the upcoming exams and come first in class.
but without a context that makes it clear that you are talking about the future, using future will seems better; by using future will in your sentence it is much clearer that you are talking about future time.
I call will in your sentence future will because the modal verb will has other uses than to talk about future time.
Best Answer
Yes, you can – but usually not as you put it. There's a pertinent explanation in Swan's Practical English Usage (260.1):
So no one would say this:
because you're essentially saying that you'd be happy to show gratitude if that will make them send you your bag, as though there'd previously been an argument between you and the other person who said:
However, your sentence would never be interpreted as such – outside the bizarre context I made up above, and even then it would be rephrased into something like
which is still weird but less weird than the original – but rather as a non-native English speaker's attempt to say what BillJ said in their comment (a closing quotation mark added):