Elementary Proofs of Two Congruences for Partitions with Odd Parts Repeated at Most Twice

In a recent article on overpartitions, Merca considered the auxiliary function $a(n)$ which counts the number of partitions of $n$ where odd parts are repeated at most twice (and there are no restrictions on the even parts). In the course of his work, Merca proved the following: For all $n\ge 0$, \begin{align*} a(4n+2) &\equiv 0 \pmod{2}, $\text{ and}$ \ a(4n+3) &\equiv 0 \pmod{2}. \end{align*} Merca then indicates that a classical proof of these congruences would be very interesting. The goal of this short note is to fulfill Merca’s request by providing two truly elementary (classical) proofs of these congruences.