YES

---- CR(R)
---- Rule Labeling [van Oostrom, RTA 2008].
R:
f(f(a())) -> f(f(f(a())))
f(f(a())) -> f(a())

Label:
[f(f(a())) -> f(f(f(a())))] = 1
[f(f(a())) -> f(a())] = 0