Usually, it’s easier for me to see how a name can be accidentally dropped from a list in the copy process. But in the case of duplications, accidental additions do not seem too much of a stretch.

The article you linked also had some very persuasive analysis regarding the oldest Septuagint manuscripts that suggest that either copyists of Luke ‘fixed’ Luke’s genealogy to match a corrupted Septuagint or copyists of the Septuagint ‘fixed’ that genealogy to match Luke.

So there are three possibilities:

1. The later Septuagint and Luke are right about Cainan and the Hebrew texts include a scribal error.

2. The Hebrew texts are right, Luke contains a scribal error that propagated into the copying of the Septuagint.

3. The Hebrew texts are right, the Septuagint contains a scribal error that propagated into the copying of Luke.

However you slice it, it’s a copyist’s error (and therefore not a problem for the doctrine of inerrancy). But I was initially tempted by #1. I now find #2 or #3 more convincing.

http://www.apologeticspress.org/articles/580

There is a substantial time interval between the initial translation of the Septuagint and the earliest extant copies of the Masoretic Text (approximately 1000 years). That’s plenty of time for a scribal error to occur and propagate.

The doctrine of inerrancy does not rule out scribal errors.