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Should we show the distribution of $6000 in 2016 tax return if we already paid taxes on it in 2015 ?"
No, since the $6,000 of taxable income was apparently "earned" in 2015 and is taxable to the grantors (i.e., owners of the trust who are entitled to that income) in that year. It is irrelevant that the funds were not withdrawn until the following year.
