I have two identical keys in an array.

Hi,

working with php 8.0.16, i am meeting a hard bug I can't really explain.
One of my script populates an array and doing a var_dump I have that :

Notice the duplicate -1 key. It should not exist as php should erase the
previous key.

array(3) {
  [-1]=>
  array(1) {
    [0]=>
    int(10585779)
  }
  [18]=>
  array(2) {
    [0]=>
    int(10585779)
    [1]=>
    int(10586274)
  }
  [-1]=>
  array(1) {
    [0]=>
    int(10586274)
  }
}


And when doing a json_encode of that, I get that :
"id_hash":{"-1":[10585779],"18":[10585779,10586274],"-1":[10586274]}


I suspected a utf8 misencoding, but it seems it's not.

Any thoughts?