Serbanuta's conjecture on attainable M-ambiguity sequences

Let $\Sigma$ be an ordered alphabet. For words $u,v\in\Sigma^*$ and a letter $a\in\Sigma$, a word is $M$-unambiguous if it is not $M$-equivalent to any distinct word, where $M$-equivalence means having the same Parikh matrix. Serbanuta's conjecture. If $uaav$ is $M$-unambiguous, then $uav$ is $M$-unambiguous. Equivalently, if $uav$ is $M$-ambiguous, then $uaav$ is also $M$-ambiguous. This conjecture concerns the possible patterns of $M$-ambiguity under insertion of a repeated letter and was proposed as an open problem by Serbanuta; its resolution is not specified in the supplied text.