I don't know how you managed to end with this query string, but let's assume you have:
index.php?asd=1&asd=2&asd=3
The you can catch it via
var_dump($_SERVER['QUERY_STRING']);
Output
string 'asd=1&asd=2&asd=3' (length=17)
Then split it by &
delimiter
var_dump(explode('&', $_SERVER['QUERY_STRING']));
Output:
array (size=3)
0 => string 'asd=1' (length=5)
1 => string 'asd=2' (length=5)
2 => string 'asd=3' (length=5)
Then split by = to find key=>value pairs.
However, this approach I find very unattractive and inefficient.
If you have control over the checkbox names, then build the query string like:
index.php?asd[]=1&asd[]=2&asd[]=3
Then
var_dump($_GET['asd']);
will return array
array (size=3)
0 => string '1' (length=1)
1 => string '2' (length=1)
2 => string '3' (length=1)