YES

   1: f(f(a())) -> f(f(f(a())))
   2: f(f(a())) -> f(a())

@Rule Labeling
--- R
   1: f(f(a())) -> f(f(f(a())))
   2: f(f(a())) -> f(a())

--- S
   1: f(f(a())) -> f(f(f(a())))
   2: f(f(a())) -> f(a())