A covered employer should have deducted employees contributions before payment of wages to the workmen. As far as employees are concerned they have already 'contributed' their share. Now it is the turn of employer to remit the same along with his share. Therefore, the question of deducting the employees share again will not arise.
Now, if the employer had OMITTED to deduct the employees share for all these 20 years, then also it is highly prejudicial to call on the employees to share their contributions for all these 20 years as the amount itself will be very high. Since the primary responsibilty lies on the employer, I do not think wise in deducting the employees' share with retrspective effect.
Let me try to get some case related to it and then I will get back to you.
Regards,
Madhu.T.K