I am a tenured professor. I recently moved to a new R1 university (less than a year) but always had a different school as my dream university. How odd is it to apply to another university given that I have been in my new university for less than a year? Is it something the new university will consider as a negative point on my application or are they just looking at the qualifications?
My suggestion is that you apply if the job description is a close fit. Not doing so passes up an opportunity that might not return. People may want to know why you want to move, but I'd save that for any actual questions. "Dream job" is a pretty good reason.
Don't worry about the place you leave behind. You've presumably done your job so there should be no complaints. The cost incurred in hiring you is a sunk cost in any case. At an R1 university, such things are not a (proportionally) major expense since the faculty is probably fairly large overall even if not in a single department. They will cope. It isn't their first rodeo.
However, expect an additional probationary term before you get tenure again. It might be two or three years and has some risk, though more in some fields than others.
As to the question in the comment to another answer, most R1 universities are bound in hiring to make a national (at least) search and to evaluate all applicants. Someone can be invited to apply but the process still has to be followed.
First, understandably, you do not say much about yourself and where you are. Academic customs differ in the US from Germany, etc.
It is not unusual for tenured faculty to apply to another university. Hiring is a costly procedure and leaving a university after only a year will leave bad feelings behind. At the dean's level, hiring someone after only a year at another university would be seen as an impolite act. This would be especially the case if it appears as if you were actively recruited, even if, as in your case, this is not the case.
That's being said, there are many exceptions justifying such a move. For example, you could be a much better fit for them.
Presumably, you know people at the other university and their estimate and advice is much more important than that of a random academic who does not know you and the two departments.