This proposal is kind of a complementary one to this one raised on Meta Stack Overflow. The TL;DR is to add a specific flag type for duplicated answers, likely including a link to the duplicated source, instead of using a manual flag.
My proposal there was to have such a flag send the answers to a special review queue, or an existing one like very low quality.
Now this is about getting the answer to a review. Your proposal is about how to handle it inside it.
One way I can think of, is that assuming the linked proposal for the flag will (magically) be accepted, the flag could automatically add such comment on the answer and deem your proposal irrelevant (in the good way, that it is no longer needed).
But let's assume this doesn't sit well with the flags mechanism (no other flag automatically posts a comment) or that it will simply wouldn't get implemented - I fully support your proposal. I too encounter many of such answers over at Stack Overflow, where some questions have tens of answers and are about a decade old. Many users repeat long existing answers, or at least very slight variations of them.
It would even be better to have a link to the duplicated source (as proposed for the flag) in that comment. This will serve as a pointer to the OP, but more importantly - it will help the following reviewers with detecting the problem and getting rid of it faster.
Another good point that was raised in a comment by Karl Knechtel, is that this could be implemented the same way duplicated questions flagging works - you get an option to choose a post that is automatically linked. The same can be done with answers
