hash_x.rb

class Hash
  def safe_invert
    lazy
      .map {|a, b| [b, a]}
      .group_by {|a| a[0]}
      .map {|a, b| [a, b.collect {|c| c[1]}]}
      .to_h
  end
end

h = {a: 1, b: 1, c: 2}

p h.invert       #=> {1=>:b, 2=>:c}
p h.safe_invert  #=> {1=>[:a, :b], 2=>[:c]}