A two-note slur over identical consecutive pitches is very likely a mis-authored tie

When a slur's exact 2-note span connects two notes of the SAME pitch, the visual result is nearly indistinguishable from a tie, and the musical effect most likely intended is what a tie represents. A slur over same-pitch notes does have a legitimate niche meaning in some traditions (a short legato re-articulation, e.g. a subtle re-bow/re-tongue), but it is rare enough that it should be flagged for confirmation.

`slurs: [[4,5]]` where `notes[4].pitch === notes[5].pitch` and neither note has `tie: true` — this is very likely the author reaching for the wrong mechanism.

Gould distinguishes tie (same pitch) from slur (typically different pitches) as the primary structural difference; the ambiguous same-pitch-slur case is acknowledged in engraving practice as rare, not the default reading.

This rule can be decided from pitches, durations and voices — no layout required. A validator can check it before anything is drawn.