Will Fellix double-book me?

No. Anything in your connected calendar blocks its slot, including events created outside Fellix, so a busy time is never offered.

No. Anything already in the connected calendar blocks the slot it occupies, so a time you are busy is never offered, including events created outside Fellix.

The appointments calendar with a struck-through event that came from the connected calendar and blocks its slot

An event from the connected calendar blocks its slot.

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