If you run twitter-to-sqlite user-timeline ... --since it's supposed to fetch Tweets those specific users tweeted since last time the command was run.

It does this by seeking out the max ID of their previous tweets:

BUT... this has a nasty flaw: if another account had retweeted one of their recent tweets the retweeted-tweet will have been loaded into the database - so we may treat that as the most recent since ID and miss a bunch of their tweets!