array has same key twice (php) - php

Question

im reading in some data and want to sort it into an array.
To be more specific, im reading in an obj model and i want to sort all edges into an array. Each edge element then should exist of an array of all faces which use this edge. So after sorting, each edge should have 2 entries existing of the two faces which use them. But after the actual sorting most edges occur twice with the same key and a single face as an entry. Here in some more details what i mean :
First Face i read in is
array(4) {
[0]=>
string(1) "f"
[1]=>
string(1) "1"
[2]=>
string(1) "2"
[3]=>
string(2) "3
which sorts the edges 1_2 , 2_3 and 1_3 into this array (value is the face which uses this edge, face number 0 )
array(3) {
["1_2"]=>
array(1) {
[0]=>
int(0)
}
["2_3"]=>
array(1) {
[0]=>
int(0)
}
["1_3"]=>
array(1) {
[0]=>
int(0)
}
When the second face is read
array(4) {
[0]=>
string(1) "f"
[1]=>
string(1) "1"
[2]=>
string(1) "3"
[3]=>
string(2) "4"
the resulting edge array looks like this :
array(6) {
["1_2"]=>
array(1) {
[0]=>
int(0)
}
["2_3"]=>
array(1) {
[0]=>
int(0)
}
["1_3"]=>
array(1) {
[0]=>
int(0)
}
["1_3"]=>
array(1) {
[0]=>
int(1)
}
["3_4"]=>
array(1) {
[0]=>
int(1)
}
["1_4"]=>
array(1) {
[0]=>
int(1)
}
}
the edge 1_3 is used by both faces, but how the heck does the key "1_3" appear twice in a single array with different values?! i expected it to look like this
["1_3"]=>
array(2) {
[0]=>
int(0)
[0]=>
int(1)
}
i do not have a clue why or how this happens, because as far as i know keys should be unique. when giving out the element "1_3" i then only recieve the latest entry ( int(1) ).
full sorting code looks like this
if ($line_temp[0]=="f"){
for ($j=0;$j<3;$j++){
$line_temp2 = explode("//",$line_temp[$j+1]);
$data["f"][$face_number][$j] = $line_temp2[0];
}
if ($data["f"][$face_number][0]<$data["f"][$face_number][1]){
$data["e"][$data["f"][$face_number][0]."_".$data["f"][$face_number][1]][]=$face_number;
} else {
$data["e"][$data["f"][$face_number][1]."_".$data["f"][$face_number][0]][]=$face_number;
}
if ($data["f"][$face_number][1]<$data["f"][$face_number][2]){
$data["e"][$data["f"][$face_number][1]."_".$data["f"][$face_number][2]][]=$face_number;
} else {
$data["e"][$data["f"][$face_number][2]."_".$data["f"][$face_number][1]][]=$face_number;
}
if ($data["f"][$face_number][0]<$data["f"][$face_number][2]){
$data["e"][$data["f"][$face_number][0]."_".$data["f"][$face_number][2]][]=$face_number;
} else {
$data["e"][$data["f"][$face_number][2]."_".$data["f"][$face_number][0]][]=$face_number;
}
var_dump($data["e"]);
var_dump($data["e"]["1_3"]);
$face_number++;
}