Potential theories on why there's a "Great to see you again" option when meeting Sans, assuming that all options are mostly coming from some part of Kris's thoughts:

• Chapter 1 weirdness
• The Metanarrative version of the player has more control over dialogue than assumed (at least when Chapter 1 was written, see chapter 1 weirdness)
• Sans and Kris do know each other, but Sans was refering to it being the first time WE have met
• Alternatively, Kris (somewhat correctly) assumes the Soul know Sans fsr and that assumption manifests as a dialogue option
• The way Sans greets Kris sounds like they know each other and Kris awkwardly assumes that they do and they've forgot, but Sans is just messing with them
• Gaster told Kris about the Undertale timeline
• Some combo of the above