# GOOD
# dup element in map
arr = [[:A, :B], [:C, :D]]; changed_arr = arr.map { |a| x = a.dup; x.delete_at(-1); x }; p changed_arr; p arr
# [[:A], [:C]]
# [[:A, :B], [:C, :D]]

# BAD
# map and delete
arr = [[:A, :B], [:C, :D]]; changed_arr = arr.map { |a| a.delete_at(-1); a }; p changed_arr; p arr
#  [[:A], [:C]]
# [[:A], [:C]]

# BAD
# dup array and map and delete
arr = [[:A, :B], [:C, :D]]; dupped_arr = arr.dup; changed_arr = dupped_arr.map { |a| a.delete_at(-1); a }; p changed_arr; p arr
# [[:A], [:C]]
# [[:A], [:C]]

Original by Github issue

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