Objects or creatures Truly Polymorphed are no longer 'currently active duplicates' and would not targeted for 'instant destruction'.
The 5e spell Simulacrum states:
If you cast this spell again, any currently active duplicates you created with this spell are instantly destroyed. (emphasis added / page 276 Player's Handbook)
It is believed this line was ...
The Rules Don't Say
Here's what the spell says:
The target’s game statistics, including mental ability scores, are replaced by the statistics of the new form. It retains its alignment and personality.
What counts as a "game statistic"? That term is never defined, so it's up to the DM to define it.
(The 5e rules have a lot of these ambiguities, ...
Polymorph works on familiars
Find familiar states:
Appearing in an unoccupied space within range, the familiar has the statistics of the chosen form, though it is a celestial, fey, or fiend (your choice) instead of a beast.
The CR is one of the statistics of the form, so the familiar has the same CR as the chosen form (which is 0 for most forms). Moreover, ...
Yes, you can cast polymorph on a familiar from find familiar.
The description of the polymorph spell simply says it targets a creature:
This spell transforms a creature that you can see within range into a new form. [...] The spell has no effect on a shapechanger or a creature with 0 hit points.
[...] The new form can be any beast whose challenge rating is ...
Be a high level spellcaster with the True Polymorph spell. Because a lower level character cannot accomplish it alone except with a magic item, and under conditions that are unlikely
Because the bulette is a monstrosity and not a beast, the regular polymorph spell will not work (it is limited to beasts); you need the 9th level true polymoprh. But that spell ...