# Avoid calling nested method two times

For a given file name path, I need to get the folder one level before it. This can be accomplished in ruby with the following code:

File.dirname(File.dirname("/a/b/c.txt"))
=> "/a"


Independently from this exact case, how can I DRY two method calls in sequence?

You should consider any DRYing efforts in terms of its effects on code clarity. I don't think that trying to reduce foo(foo(bar)) is worth the reduced clarity. Now, if we're trying to reduce foo(foo(foo(foo(foo(bar))))), that's something else. One way you can do that is using reduce:
5.times.reduce(1 << 64) { |x,| Math.sqrt(x) }