Dave Peterson wrote...
If you change Harlan's formula to:
=IF(MOD(ROW(),2),INDEX(A$1:A$400,INT((ROW()+1)/2)),NA())

You can use the Edit|Goto|special|formulas and leave Errors
selected (and unselect everything else).
....

I screwed that up.

There's another alternative. Copy A1:A400 into B1:B400, enter =ROW()
in C1, then double click on C1's fill handle to fill it into C2:C400.
Copy C1:C400 into C401:C800. With C401:C800 still selected and C401
the active cell, type

=C1+0.5

and press [Ctrl]+[Enter], select B1:C800 and run the menu command Data
Sort, sorting the range on column C in ascending order. Once sorted,
clear C1:C800.