How to combine hashes without data loss

hash1={"a"=>1,"b"=>2,"c"=>3}
=> {"a"=>1, "b"=>2, "c"=>3}

hash2={"c"=>4, "d"=>5,"a"=>3}
=> {"a"=>3, "c"=>4, "d"=>5}

hash1.merge(hash2)
=> {"a"=>3, "b"=>2, "c"=>4, "d"=>5} #"a"=>1 & "c"=>3 items in hash1 were lost

hash1.merge(hash2){|key, oldval, newval| [*oldval].to_a + [*newval].to_a }
=> {"a"=>[1, 3], "b"=>2, "c"=>[3, 4], "d"=>5}