A tie's next note must be the identical single pitch

A tie connects two noteheads of the same written pitch (same letter name, same accidental, same octave) that sound as one continuous note.

Gould: 'Ties join two notes of the same pitch.' Read states the tied notes must be 'on the same line or space and preceded by the same accidental (or lack of one).' Ross concurs.

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