Find count of occurrences of a key in an array that appears only once?

Find count of occurrences of a key in an array that appears only once?

Hi

i have the following array

test ={  [id:1,subject:math],  [id:2,subject:math],   [id:1,subject:science],    [id:3,subject:science],  }

how can I find the key that appears only once like..   [id:2,subject:math] ,  [id:3,subject:science].

I need to output the key id value 2 and 3.

Thank you