Each sector must appear twice per round, plus 2 specials
For each set of three rounds, the fifteen combinations of sector to sector runs and the six sectors for specials must each appear once.
Each number (11, 44, S2 etc.) must be used once in each set of three rounds.
No race can be repeated across all 6 rounds, taking double and triple number destinations into account (so 11 - 33 and 11 - 34 is a repeat if 33 and 34 are the same location).
The above could be be made an option, for now you need to de-duplicate town names (e.g. London1, London2) to turn it off.
The system doesn't check for minimum race length (regenerate races if violated) - beware!