Hi,
i have the same issue. And i'm pretty sure this wasnt the case in 0.9.9.
Status: Sigil not open
0.9.9 in finder clicking on "open with" opened sigil twice, one instance with the "empty" standard ebook and onme instance with the choosen epub.
0.9.17 in finder clicking on "open with" opens sigil once with then "empty" statdard ebook.
Sigil already running
0.9.9 "open with" works as expcted
0.9.17 "open with" works as expected
The same behavior with double clicking on a eoub file. This defintily worked in 0.9.9, in 0.9.17 not
For me its not a big issue as i have Sigil mostly running all the time as i'm lazy, not closing Apps nor shuting down the computer. So Sigil is running most of the time and everything is fine.