insert EOD if value in Field 1 change

Shell Programming and Scripting
1

How to create simple awk program data such as below:
input:
1 23
1 34
1 12
2 10
2 11
2 12
3 11
3 12
3 13
expected output:
1 23
1 34
1 12
EOD
2 10
2 11
2 12
EOD
3 11
3 12
3 13
EOD

Thanks

2

Is this a homework requirement?

Regards

3

Yes, it is. So please help. Thanks