Tying and untying a double knot on Shabbos:
[Due to that we rule stringently in the above dispute to prohibit tying and untying professional knots in all cases] we are therefore accustomed to be careful not to tie or untie any knot which is a double knot one on top the other even if made [in order] to undo on that same day. [See Q&A]
The reason is: because we are not experts as to which knots are considered professional of which are [thus] forbidden to tie and untie even if not made to last at all, and [thus] any strong knot which is very tightly tied there is doubt that perhaps this is considered a professional [knot], and [thus since] a double knot is also a strong knot [therefore we are stringent].
Untying a double knot in a scenario that involves pain: Nevertheless in a scenario [that involves] pain one need not be strict regarding [the dispute of the] Rabbinical [prohibition] and one is [thus] allowed to untie it if [the knot] is not made to last at all in a way that it would for certain be allowed to be done [if it were an] amateur [knot], being that according to the latter opinion it is allowed even if it is certainly a professional [knot].
May one tie or untie a loose double knot?
Yes. If it was tied to undo that day it may be untied, as a knot can only be considered a professional knot when it is strong and tight.
However the Misheneh Berurah learns that so long as it will not be undone on its own, even if it can be undone with one hand, then [a double knot] is considered a professional knot.
 However the Shareiy Teshuvah 1 brings from the Bircheiy Yosef that their custom is to allow tying a double knot to undo that day as it is considered an amateur knot, and its only a minority opinion which argues this.
 Ketzos Hashulchan 123 footnote 5 learns that this allowance to undo a knot in a case of pain only applies to a double knot, being that there is doubt as to whether it at all is considered professional. However by a knot that is certainly professional, then despite that this too is disputed whether it is Rabbinically forbidden when made to last less than one day, nevertheless one is to be stringent even in a case of pain. This is also implied from the ending wording of Admur “being that according to the latter opinion it is allowed even if it is certainly a professional [knot]”. Thus implying that his allowance was only referring to the last case mentioned which involved a double knot which is a questionable professional knot.
 Ketzos Hashulchan 123 footnote 4. Kaf Hachayim 317/22. Based on 317/ 2
 317/1-see Biur Halacha there,
So too seems to learn the Kitzur Shulchan Aruch who prohibits double knotting a scarf.