Let $(X_{n,t})_{t=1}^{\infty}$ be a stationary absolutely regular sequence of
real random variables with the distribution dependent on the number~$n$. The
paper presents the sufficient conditions for the asymptotic normality (for
$n\to\infty$ and common centering and normalization) of the distribution of the
second-order $U$-statistic of $X_{n,1},\ldots,X_{n,n}$ with a kernel also
dependent on $n$. To analyze sums of dependent random variables with rare
strong dependencies the proof uses the approach that was proposed by Svante
Janson in 1988 and upgraded by Mikhailov in 1991 and Tikhomirova and Chistyakov
in 2015.