If on Shabbos, a Kattan ties one knot on the shoes of a father and then a second Kattan ties another knot on the same shoes, so now the shoes have a double knot, was any Melacha performed? Are the shoes permissible to be worn by the father? Thank you.
The second son that tied the second knot did the melacha, because he now created a full knot. Regarding the father, he is allowed to wear the shoes because they are usable even if they are not tied.
Maharsham 6-34, Shevet Halevi 8-173(4), Shemiras Shabbos Khilchoso 15-54.