Hi Mr. Jayanta,
Dual employment in India is generally restricted under various labour laws and by the terms of most employment contracts. In cases of misconduct, an employer is required to follow the principles of natural justice, which include issuing a show-cause notice, conducting a fair enquiry, and giving the employee an opportunity to defend themselves. If your current employer terminated you abruptly on the basis of the previous employer’s complaint without giving you a hearing or conducting an enquiry, such termination can be challenged as procedurally unfair under the Industrial Disputes Act, 1947 (if you are a ‘workman’) or through a civil suit for breach of contract (if you are not).
You can also dispute the demand for refund of the full salary if there was no contractual clause authorising such recovery, or if the employer cannot show that they suffered a quantifiable loss due to your actions. Courts have, in certain cases, ruled in favour of employees when employers skipped due process, even if the misconduct was admitted. Your legal strategy would be to focus on the procedural lapses and lack of due process, rather than contesting the fact of dual employment itself.
If you wish to pursue the matter, connect with me on adv.vishesh@icloud.com for further discussion.