Original variable changes outside the recursive function's scope even though I duplicated it

I come from a python and C++ background. The original variable is real_array_slice and real_slice.Input array: [2.799999952316284, -0.40000003576278687, -0.40000003576278687, -0.3999999761581421, -0.3999999761581421, -0.3999999761581421, -0.4000000059604645, -0.40000003576278687][2.799999952316284, -0.40000003576278687, -0.40000003576278687, -0.3999999761581421, -0.3999999761581421, -0.3999999761581421, -0.4000000059604645, -0.40000003576278687]

Function is called and prints the correct input array:

print(real_array_slice)real_array,imag_array = fft(real_array_slice,imag_array_slice,sign)

Inside that function the printed array changes due to recursive calls, but this should be impossible because the variable is being duplicated:

def fft(real_slice = [],imag_slice = [],sign)  console.clear  print real_slice  evens_real, odds_real = fft_recursion(real_slice.dup,sign)  evens_imag, odds_imag = fft_recursion(imag_slice.dup,sign)

For further investigation, the recursive function:

def fft_recursion(slice,sign)    return slice if slice.size <= 1    evens_odds = slice.partition.with_index{|_,i| i.even?}    if sign == -1 #FFT    evens, odds = evens_odds.map{|even_odd| fft(even_odd, sign)*2}    else #IFFT    evens, odds = evens_odds.map{|even_odd| fft(even_odd,sign)}    endend

I tried keeping the recursive function and the main function together, but the same issue occurred.The array should stay the same outside the recursive function calls.