How do I not let squares be overwritten in tic-tac-toe?

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Ruby

I wrote a tic-tac-toe program. I wrote code so that the user can't overwrite a taken square. It does't let "o" overwrite one of the taken "x" square but for some reason "x" can overwrite a taken "o" square.

Here is the if-statement where I wrote this code (let me know if you want the rest of the program):

 def make_move
                puts "Enter x coordinate"
                x=gets.to_i
                puts "Enter y coordinate"
                y=gets.to_i

if y==1 && x==1 && !@board[1].match(/[xy]/)
        @board[1]="1  _"+@turn+"|"
elsif y==2 && x==1 && !@board[2].match(/[xy]/)
        @board[2]="_"+@turn
elsif  y==3 && x==1 && !@board[3].match(/[xy]/)
        @board[3]="|_"+@turn
elsif y==1 && x==2 && !@board[4].match(/[xy]/)
        @board[4]="\n2  _"+@turn+"|"
elsif y==2 && x==2 && !@board[5].match(/[xy]/)
        @board[5]="_"+@turn
elsif y==3 && x==2 && !@board[6].match(/[xy]/)
        @board[6]="|_"+@turn
elsif y==1 && x==3 && !@board[7].match(/[xy]/)
        @board[7]="\n3   "+@turn+"|"
elsif y==2 && x==3 && !@board[8].match(/[xy]/)
        @board[8]=" "+@turn
elsif y==3 && x==3 && !@board[9].match(/[xy]/)
        @board[9]="| "+@turn+" \n"
else
        puts "You either entered an invalid coordinate or a a taken square. Please enter another coordinate"
        make_move
end