The Union Government Law supersedes the state in case of ambiguity ... What exactly is the status of polygamy for hindus in Goa?
The Union law need not necessarily supersede the state law. Article 254(2) of the indian constitution allows state laws to exist, even when it conflicts with central laws, if the centre gives its official consent for it by Presidential assent and the law is promulgated. The Supreme Court of India has also affirmed this in its judgement in M. Karunanidhi vs Union Of India, 1979:
(iv) Where, however, a law made by the State Legislature on a subject covered by the Concurrent List is inconsistent with or repugnant to a previous law made by Parliament, then such a law can be protected by obtaining the assent of the President under Art 254(2) of the Constitution. The result of obtaining the assent of the President would be that so far as the State Act is concerned, it will prevail in the State and over-rule the
provisions of the Central Act in their applicability to the State only. Such a state of affairs will exist only until Parliament may at any time make a law adding to, or amending, varying or repealing the law made by the State Legislature under the proviso to Art. 254.
As India is yet to adopt a Common or Uniform Civil Code as envisioned in Article 44 of the Indian constitution, and the Goan civil laws are largely a common civil law (with exceptions like the one you noted) the Union government has allowed these laws to exist, without contesting it, in the hope the state legislature will eventually reform it (and this has indeed been happening but at a slow pace) or Parliament will adopt a UCC that will make these laws redundant. Moreover, since no constitution bench has examined these specific provisions of Goan civil laws and struck it down, they do indeed continue to be legal and applicable only in the state of Goa and Daman and Diu.
Though the makers of the indian constitution favoured creating a single law, applicable to all citizens on civil issues, like marriage, divorce, inheritance, adoption etc., they couldn't get a political consensus for it. And thus India decided to continue, with some changes, the old British colonial practice of community specific personal laws.
Ambedkar in his speech in the Constituent Assembly had said, "No one need be apprehensive that if the State has the power, the State will immediately proceed to execute…that power in a manner may be found to be objectionable by the Muslims or by the Christians or by any other community. I think it would be a mad government if it did so." (Source: What is Uniform Civil Code).
This has resulted in multiple personal laws like Hindu Marriage Act, 1955 and Hindu Succession Act, 1956, Hindu Minority and Guardianship Act, 1956, Hindu Adoption and Maintenance Act, 1956, Indian Succession Act, 1925, Shariat (Application) Act, 1937 and Dissolution of the Muslim Marriage Act, 1939, and neutral ones like the Special Marriages Act etc being in vogue today.
Despite this compromise, the constitution makers were all in consensus that reforms in cultural and traditional practice of the various communities were necessary. And India should ultimately have a common civil code applicable to all Indians in the future. This was emphasised by including Article 44 in the Indian constitution (pdf) that says:
Uniform civil code for the citizens. — The State shall endeavour to secure for the citizens a uniform civil code throughout the territory of India.
India has a federal system of governance and on some subjects both the state legislature and the central legislature can make laws. The civil issues:
Marriage and divorce; infants and minors; adoption; wills, intestacy and succession; joint family and partition ...
where UCC is desired, is in the concurrent list meaning that states too are allowed to make laws on these subjects.
The modern state of Goa and UT Daman and Diu were once Portuguese territories. So unlike the rest of India where British laws applied, the Portuguese Civil Code, 1867 (pdf) was in practice here for decades. And, as was the practice of Portuguese colonial rule, the code had been extended to protect the "practice and customs of the locals":
Article 9 – The Government is empowered to extend the Civil Code to the overseas provinces, after hearing competent agencies and after making modifications, which are required by special circumstances of the provinces.
- In the exercise of this permission, the Government published the Decree of 18-11-1869 extending the Code to the overseas provinces safeguarding the usages and customs of the natives of the New Conquests. The Code came into force therein on 01st July, 1870. Subsequently by Decree of 16th December, 1880, it ordered safeguarding in favour of the gentile Hindus of Goa without distinction of Old and New Conquests, their special and peculiar usages and customs reviewed and codified by this decree.
Thus, as pointed out in this answer, the specific section of the Code of Gentile Hindu Usages and Customs of Goa that allows polygamy for Hindus are:
The marriage contracted by a male Hindu by simultaneous polygamy shall not produce civil effects, except in the following cases only:
(1) Absolute absence of issues by the wife of the previous marriage until she attains the age of 25 years.
(2) Absolute absence of male issue, the previous wife having completed 30 years of age, and being of lower age, ten years having elapsed from the last pregnancy.
(3) Separation on any legal grounds when proceedings from the wife, and there being no male issue.
(4) Dissolution of previous marriage ...
(Basically, the law unfairly permits a Hindu man to marry again in case his current wife can't produce a male heir for him).
When India freed Goa and Daman and Diu from Portuguese rule, and integrated it with India, they incorporated and retained many of the old Portuguese laws including the Portuguese civil code. Despite ongoing reviews of the old laws, these aspects of family laws have not been recalled or re-codified and thus are still legally applicable.
As the questioner pointed out, this state law permitting polygamy under specific circumstances is in contradiction with the the Hindu Marriage Act, 1955 that legally bars Hindus from a bigamous or polygamous relationship:
Conditions for a Hindu marriage. - a marriage may be solemnized between any two Hindus, if the following conditions are fulfilled, namely: (i) neither party has a spouse living at the time of the marriage; ...
Void marriages. - Any marriage solemnized after the commencement of this Act shall be null and void and may, on a petition presented by either party thereto, [against the other party], be so declared by a decree of nullity if it contravenes any one of the conditions specified in clauses (i), (iv) and (v) of section 5.
Punishment of bigamy. - Any marriage between two Hindus solemnized after the commencement of this Act is void if at the date of such marriage either party had a husband or wife living; and the provisions of sections 494 and 495 of the Indian Penal Code shall apply accordingly.
Moreover, Section 494 in The Indian Penal Code also makes it a punishable offence:
- Marrying again during lifetime of husband or wife. — Whoever, having a husband or wife living, marries in any case in which such marriage is void by reason of its taking place during the life of such husband or wife, shall be punished with imprisonment of either description for a term which may extend to seven years, and shall also be liable to fine.
Article 254 of the constitution of India is meant to address what should happen in case of such conflicts when both the state and the central legislature make different laws on the same subject in the concurrent list,
Inconsistency between laws made by Parliament and laws made by the Legislatures of States.
(1) If any provision of a law made by the Legislature of a State is repugnant to any provision of a law made by Parliament which Parliament is competent to enact, or to any provision of an existing law with respect to one of the matters enumerated in the Concurrent List, then, subject to the provisions of clause (2), the law made by Parliament, whether passed before or after the law made by the Legislature of such State, or, as the case may be, the existing law, shall prevail and the law made by the Legislature of the State shall, to the extent of the repugnancy, be void.
(2) Where a law made by the Legislature of a State with respect to one of the matters enumerated in the Concurrent List contains any provision repugnant to the provisions of an earlier law made by Parliament or an existing law with respect to that matter, then, the law so made by the Legislature of such State shall, if it has been reserved for the consideration of the President and has received his assent, prevail in that State:
Provided that nothing in this clause shall prevent Parliament from enacting at any time any law with respect to the same matter including a law adding to, amending, varying or repealing the law so made by the Legislature of the State.
Since the Goan laws have not been challenged and over-ruled in any constitutional court and has been promulgated as law with the assent of the President of India and have been in practice since then, Article 254 (2) applies for this particular case and thus the Goan law prevails in Goa and Daman and Diu.
Moreover, it can be argued that the legal standing of the Goan Civil Code is strengthened by Article 44 of the Indian constitution, as Goa is the only Indian state currently honouring it (even if by a faulty quasi-"Uniform Civil Code") and Parliament is yet to make any Common / Uniform Civil Law.
(All said, in my personal opinion the courts can indeed strike down that particular provision related to allowing polygamy if it is challenged in the Supreme Court today because it no longer meets the expanded principles it defined for evaluating laws under the concept of repugnancy.)