How to extract certain records froma csv file

Hi,
First I tried posting this earlier today and do not see it, so if it is some how doubled posted I am sorry; I did not know what else AI can do here, but re-post it.
I have a CSV file where the records are identified by a record ID. I want toe extract only the records that have two of these IDs.
Example I can have something as follows:
1... data
1... data
1... data
1... data
1... data
2... data
2... data
2... data
3... data
3... data
3... data
4... data
4... data
4... data
5... data
5... data
5... data
6... data
6... data
6... data
6... data

Now lets say I only want to copy the 1 and 5. keep in mind that the file, that I work with is extremely large and each of these records goes for hundreds of rows.
How can I find the begging and the end of each record number and just copy all the records for 1 and 5.

Thank you