The question is definitely not trivial, but is not currently at research level because the answer has been known for a long time. A good source available online here is the 2010 ICM write-up by Shrawan Kumar. He attributes the result to the late Bert Kostant in a seminal 1959 paper on weight multiplicities. See Proposition (3.2) in Kumar's recent survey, where there is also a natural further refinement giving an upper bound on the multiplicity of each irreducible summand of such a tensor product.
[By the way, I included a version of this result as Exercise 24.12 in my 1972 graduate text on Lie algebras, though for some reason I can't recall now the exercise stated that it should be deduced from Steinberg's tensor product formula. That seems misleading.]