Select 0 item from n identical items (only in 1 way), rest n items from n distict items in $\binom{n}{n}$ways
OR
Select 1 item from n identical items(only in 1 way) , rest n-1 items from n distict items in $\binom{n}{n-1}$ ways
OR
Select 2 item from n identical items (only in 1 way), rest n-1 items from n distict items in $\binom{n}{n-2}$ ways
.
.
.
OR
Select n item from n identical items (only in 1 way), rest 0 items from n distict items in $\binom{n}{0}$ways
So. total = nCn + nC(n-1) + nC(n-2) + ...+ nC0 = 2n