How can I do a second graduation if the post graduation passing year is before the new graduation ??

You can do the graduation with the same subjects from a different university, in distance mode. It will be completed in the year 2023.
When you make a CV, you should show both graduations. You have to mention the year of the passing of both the degrees. Otherwise, if you show only the recent one then the question will arise how graduation after Post Graduation. The organisation may not consider your application. When you show both they will take your second graduation for consideration if you get a higher percentage in that.
You can also write in your CV that you have taken graduation to another university. I don't foresee any problem in that. So you can go ahead.

There was a time when there was no provision of improvement examinations for improving one's score in a degree. Later supplementary examinations were added and it relieved many students and they got through the supplementary exam though in the normal examinations they had failed. Now we are in the modern era where you are entitled to go for improvement in your degree by again attempting the same examination may be from a different college or through online mode. If you score better in the present attempt then you have to show both the degrees in your CV one that is the earlier one and the other the present one where score is better. If you do not get a good score this time then forget about the new one and just go ahead with the older one. There is nothing wrong in this proposition. Only issue is you are sacrificing 2 years of your valuable prime time when others are making their careers elsewhere. That itself is a loss to you in terms of the time which is also an important factor in our lives. Anyway go ahead for your improvement endeavour and I do not see any problem in that.

There was no scope of attending the second graduation despite the bad performance earlier in our academic system but now it is possible if one is not satisfied with the present score of the graduation. It does not matter that even a post graduate degree has been taken up.
Improvements in the scorings is sometimes essential since in many organisations while filling up especially the executive posts, selection committee would verify the percentage of each examination and the percentage of each examination should not fall below 60 percent while recruiting the post of Management trainee , administration in SAIL, DVC, ONGC etc. Similarly there are other organisations considering consistency in respect of performance of the aspirants. In NET examination also, there is a pattern of at least securing 55 percent in the qualifying examination, but any aspirant having qualified in NET in the written test, his graduation mark falling below 50 percent would attract the Board otherwise and his posting by way of selection in the lecturership may not in a place of his liking.
Hence improvement in the marks of graduation would be in the line of enhancing of one's employability.
You would be required to meet your Principal from where you have graduated and submit your application stating the cause of improvement of marks of your graduation and your application would be forwarded to the university with the remarks of the Principal. You would be required to submit prescribed fee along with the application.
Thus if you are able to secure beyond 60 percent, your attempt would be worthwhile and your improved marking can be shown in the appropriate places such as in consideration of such posts calling for consistency of marks, your further persual of academic research etc.
If you fail to achieve a better scoring, your last performance in terms of graduation examination would be taken into final in case of selection of your candidature.