If one neglects his work, but still gets paid for it, can he double the amount of his work in the next month to make it up in effort and work to compensate, would his past gains be halal for him ?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

The employee must carry out the work entrusted to him without negligence, according to what is agreed upon between him and the employer. Allah the Almighty says (what means): {O you who have believed, fulfill [all] contracts} [Quran 5:1]. And the prophet's saying as well, “Muslims must keep to the conditions they have made, except for a condition which makes unlawful something which is lawful or makes lawful something which is unlawful.” [At-Tirmidhi, who classed it as Hasan Saheeh]

If the employee fell short and did not do all the work entrusted to him, then he is not entitled to the salary except to the extent of the work he has accomplished. It is not permissible for him to take what exceeds the deserved amount of his work, because it is considered as money that was taken unjustly. If he took it, then he must return it to the employer. As for what you have mentioned in the question of "doubling the amount of his work in the next month" to compensate for it, then if you mean that he does an extra work than what is agreed upon in order to deserve that extra amount, then we do not see it acquitting him from liability, because the extra work is not originally contracted for and it is a compensation that lacks the mutual consent of the two parties, so it is not permissible for him to possess the previous money that he did not deserve by work which was not contracted for with the employer, but rather he should return the money to the employer or agree with him to increase the work in return for that money taken unjustly.

Allah knows best.