There are already build-in names for physical dimensions, you can find them here
Use Frequency, it looks nicer and is more explicit
julia> using Unitful; using Unitful: s, Hz, Frequency
julia> f(x::Frequency) = 2x
f (generic function with 1 method)
julia> f(2Hz)
4 Hz
julia> f(1/2s)
1.0 s^-1