In the four questions, we ask "on all other nights, we don't dip, even once, but on this night, we dip twice"
How can we ask this if we dip challah in salt on Shabbat (and other days)?
The Lubavitcher Rebbe (Sichos Kodesh 5741 vol 3. pg. 408) explains that the meaning of "טיבול" (dipping) here is in a liquid that necessitates washing hands prior such as the Karpas in salt-water, or the Marror in Charoses (that contains wine). However, dipping bread in dry salt is not considered "טיבול".
[In that talk, the Rebbe made reference to a Torah journal that had been just been printed (Yeshivas Tomchei Temimim Ocean Parkway Gilyon 5 Haaroh 16), which quoted him as explaining that on a regular Friday night one does not have to dip the Challah into salt, but could rather sprinkle the salt onto the Challah. He commented that this answer is not necessary (as above), but since it was published in his name, he probably had said it "l'richvcha demilsa"].