The "duplicated lines density" warning triggers on this patch: https://gerrit.wikimedia.org/r/c/mediawiki/core/+/658312
The issue seems to be that we changed a message signature, which means making the same one line change in many places. Many of these lines will look the same (parameter lists of subclasses, or invocations).
It seems like the warning should not trigger of the repeated chunks are very small (one line), or if the total number of lines is low.