Remove a superfluous pair of []s in cond collapser
Switch to a sane deduplication system

The original naive approach from 8e40a627f was utterly braindead and prone
to false positives. Fix this for good
Deduplicate (and stabilize) the result of _collapse_cond

Among other things set the stage to a fix of RT#98161 (later commit)
Make sure cond collapser works case insensitively
Make sure empty cond collapser works on all positions

Adds another round of sql stabilization (akin to 5268b1da6)
Fix condition collapser corrupting -X operators

This is (fingercross) the complete fix for RT#98161