ok, found a hidden treasure:

create a new sketch with an additional tab.
add same variable declaration into both main and second tab, ie.:
"float foo;"
save somewhere .. maybe not needed.
delete the additional tab.
press run, although the second tab is gone the "duplicate local variable .." error will appear.

might be the same issue: you cannot recreate a tab named the same as the one you deleted
before, it says that that file already exists although it's not there anymore (finder).

F

Fixed for 1.0.8, thanks for the report.