how can i run over a 2d array perimeter and check if there is a number that repet itself?

how can i run over a 2d array perimeter and check if there is a number that repet itself?

int  matrix[N][N]={  {4,2,1,3,7},
                     {29,7,1,17,13},
                     {26,28,13,4,16},
                     {9,14,22,11,11},
                     {20,6,47,31,17}};

how can i check if for exmaple number 4 (mat[0][0]) appears in perimeter = 0?

perimeter = 0 == {4,2,1,3,7}
                 {29,-,-,-13}
                 {26,-,-,-,16}
                 {9,-,-,-,11}
                 {20,6,47,31,17}};

i need to check every number in that perimeter if its repet it self and check the folowing perimeter the same with its own numbers.

perimeter = 1 == {-,-,-,-,-}
                 {-,7,1,17,-}
                 {-,28,-,4,-}
                 {-,14,22,11,-}
                 {-,-,-,-,-}

if number 7 appear more than once in that perimeter return -1 for exmaple or number 1 or 17 and etc...

I would really appreciate the help, thanks in advance!!