what does this double mean?

[John Kruiniger]

W N E S
1H X no no
1S X

West opens 1H, E passes and S passes thereby converting the first X to
penalties.

S obviously has a bunch of hearts and enough values to think they're
taking 1H down.

W runs to 1S and N doubles again.

What does North's second double mean?

[James Dow Allen]

Penalties.
South's penalty pass conveys the message "despite that 1-level penalties are seldom the best spot
this hand is probably an exception." North respects this message and doubles 1S for penalties
with good spades. It is easy to construct "normal" hands where North-South have no game
but can defeat either 1H or 1S by 2 tricks or more.

If the double is not for penalties, North will often have no logical call at all.

Cheers,
James

[judyo...@gmail.com]

The penalty pass should be avoided with a side suit as good as Qxxx, even when holding the expected QJxxxx in the suit opened. Maybe "should" is too weak.

So "bid your best suit" is not an appropriate meaning for the second double. They have a hand too good to overcall.