If I do my graduation again, will the previous graduation certificate be valid?

You can do again a graduation either from the same university or a different university. If it is a different university you can do in the same subject and you can keep both the degrees intact. If it is the same university you can retain the degree with a higher percentage. That is not an issue. Till you get the new degree. the old degree will be in force. My suggestion is to go to a different university or a different subject in the same university. For example, if you have done BA mathematics from a university and again if you want to do the same degree go to a different university. Otherwise, you can select a different subject combination in BA in the same university. In such a case you will not lose time and you can continue applying for any job based on the existing qualification.
If you do a degree in a different subject or different university it will be like a double degree only.

Yes, you can do graduation again with the same subjects requesting the university to allow you for the reappearance in such subjects because of your less scorings and this examination would be treated as an effort for the improvement purpose. In that situation, you need to write an application to your Principal from where you have passed out the said examination stating the reasons for your reappearance for such an examination. He would forward your application to the university for approval and once the same is approved by the vice chancellor, you will have to fill up the application form along with the prescribed fee for such an examination.
If there is improvement in the marks by way of writing of papers in the next time, this degree will be valid and old one is automatically rejected. The university will issue you a certificate indicating that it is your second time to reappear for the improvement purposes.
Your previous degree will get cancelled but there is a catch in this situation, if your scorings are less in the second time, you cannot claim the marks of earlier examination. You have to be satisfied with the present performance. You need to keep this point into consideration.

There is a provision of improvement examination for doing graduation again either in the same subject or fresh in some other subject and generally the institutions give this chance up to 4 years of passing the earlier graduation. It can be done from the same institution or some other one. Generally the working people prefer to do it from an open university in correspondence mode. So one can attempt it and if one secures more marks then can use that as the fresh graduation degree. Such degrees are issued with a remark that it was done by the student for improvement purposes. The problem with these provisions is that even if one secures more marks the second time it is clear that the year of the getting it would show that it was a second attempt by the students and it would definitely be a negative point in getting selection or promotion anywhere in any organisation. Other point is that for doing a graduation degree second time one would be spending three precious years of ones prime time and that is a long period if we see it with respect to the rewards achieved in lieu of that. So some people prefer to acquire training in some skill based and practical courses of commercial and business interests rather than going for an improvement graduation attempt.