How can get Ruby to tell me when I'm overwriting an existing key in my
hash?

For example,

   h = { "key1" => [6, 44, 12],
         "key2" => [8, 1],
          .
          .
       }

The hash could get big enough where I accidentally add a new entry,

         "key1" => [99, 3]

when what I needed to do was add [99, 3] to key1's array. I won't get
any warning that the first occurrence of key1 is being replaced.