can I hv 2 properties on my name

 Yoy have every Right to possess more than one  property in your name in any state in India...Indian Law is universal .Right to property was former a fundamental right Under Article 32 of Constitution of India.So There is no restriction on the no of property u may own. 

You should ask the builder for reasons and when you knew about this from the begginning you should have made it clear to them that u do posses more than one property...Ignorance of a condition is not excuse just like ignorance of law is no excuse.Since its gone in litigation u have two ways to solve this...

1. Either convince the Builder to not make an issue of the truth which u will have to disclose.Because affadavit is an oath that you take and after finding the truth the builder can sue you for giving fabricated documents which is a non cognizable offence  

OR

2. Tell the court You were unaware of the fact that you cannot have two properties or simply ask for refund

 Such a condition is against article 19 which is right to life and liberty you have the liberty to buy as many properties you want in your name so u can tell the court....but also keep in mind that before giving the payment u were aware of the condition? so the court doesn help who sleep over thier rights...you should have asked the builder the reason for setting such condition.

 Which bye-laws government coop hsg societies in Delhi?

In Maharashtra, before the new model bye-laws were promulgated, under the old bye-laws, one member could not own two flats without specific permission of the  General Body and Registrar. That has changed now. There is no restriction. 

What is the position in Delhi? Check out.

Thanks alot for your guidence. Now I have made the payment can I gift the existing flat to my mother and mention in the gift deed that whenever the case is over I will pay the required stamp duty to transfer in your name. Please guide

 Payment of stamp duty is not conditional upon decision of the court. If there is an agreement, stamp duty has to be paid before registration. There are no if and buts.