The 20th Anniversary edition of Wraith: the Oblivion describes the mechanic for learning unknown arts at the same rating as a known art (e.g. learning an initiate art, when the common art is known) thusly:
An eligible character who already knows a common art for a given Arcanos rating may also learn the initiate art for the same rating or vice versa. This carries the same experience-point cost as the original dot for that level of the Arcanos.
The section on spending XP gives the following costs related to arcanoi:
|Arcanos||current rating * 3|
From the first paragraph, the "current rating" when there's existing art is not the rating of the arcanos that's written on the character sheet, but the what the rating would have been when first increasing the rating to the art being learned; in other words, 1 less than the rating to be learned.
This seems to leave out one case. What's the XP cost for learning a 1 dot art for a known arcanoi (ignoring any discounts for a guild tutor)?
For example, suppose a wraith had two dots in Argos, both in the initiate arts (Enshroud, Wayfarer's Resolve), and the player wanted the character to learn the 1-dot common art (Weather Eye). It's not a new arcanos, so it shouldn't be the 7 cost. Additionally, having a 1-dot art cost more than a 2 dot art (Phantom Wings would cost 3) doesn't seem reasonable. The only other rule would put the cost at 0 xp (the "current rating", which would be 0 for a 1-dot art, times 3), which also isn't reasonable.