How to merge duplicate keys in a JSON using python

Question

I have a JSON as shown

myjson = {
   "key1" = "value1",
   "key2" = "value2",
   "key3" = "value3",
   "key3" = "value4",
   "key4" = "value5"
}

Case where both the "key" and "value" are dynamic with a condition where "values" are unique.

I want to merger duplicate "keys" and get result "myjson" as shown

myjson = {
   "key1" = "value1",
   "key2" = "value2",
   "key3" = ["value3", "value4"],
   "key4" = "value5"
}

Any way?