Do you have electric heaters on those circuit? I
would suspect that someone ganged the breakers
with two wires instead of using one heavy wire.

He said:
gfi socket as bathroom had no socket only only light switch.
discovered that all upstairs (bathroom and 3 bedrooms) were on 2 (two)
20 amp breakers that were ganged.

Doesn't sound like these are heating circuits, but General purpose ones.

Either there's a shared neutral, or the wiring from both circuits goes into
a common junction box (which doesn't require ganging, but some people do for
safety), or someone was stupid -- after all they missized the breaker so
they could have mis-applied it as well.

-Tim