Is marriage with a woman who has not taken divorce from earlier marriage valid?

A.You don't need to divorce her as your marriage is void and invalid as per Section 11 of Hindu Marriage Act, 1955. You are not married to this woman and hence you can just ignore this marriage as null and void.

Section 11 in The Hindu Marriage Act, 1955
11 Void marriages. —Any marriage solemnised after the commencement of this Act shall be null and void and may, on a petition presented by either party thereto 11 [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.

Section 5 in The Hindu Marriage Act, 1955
5 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;
2 [(ii) at the time of the marriage, neither party—
(a) is incapable of giving a valid consent to it in consequence of unsoundness of mind; or
(b) though capable of giving a valid consent, has been suffering from mental disorder of such a kind or to such an extent as to be unfit for marriage and the procreation of children; or
(c) has been subject to recurrent attacks of insanity 3 [***];]
(iii) the bridegroom has completed the age of 4 [twenty-one years] and the bride, the age of 5 [eighteen years] at the time of the marriage;
(iv) the parties are not within the degrees of prohibited relationship unless the custom or usage governing each of them permits of a marriage between the two;
(v) the parties are not sapindas of each other, unless the custom or usage governing each of them permits of a marriage between the two;
6 [***]
(i) A marriage between a Hindu man who converted as Christian and a Christian lady in a Hindu form is not a valid marriage. According to section 5 of the Act marriage can be solemnised between two Hindus; M. Vijayakumari v. K. Devabalan, AIR 2003 Ker 363.
(ii) To draw an inference merely from the fact that the spouses had no co-habitation for a short period of about a month, is neither reasonable nor permissible. To brand the wife as unfit for marriage and procreation of children on account of the mental disorder, it needs to be established that the ailment suffered by her is of such a kind or such an extent that it is impossible for her to lead a normal married life; R. Lakshmi Narayan v. Santhi, AIR 2001 SC 2110.

A.Hi,
Yes, you are eligible for divorce with her since the marriage is not a valid one in the first place. As per S.5 of the HMA, a valid marriage consists of monogamy only. If the person has a living spouse at the time of second marriage then the marriage is illegal.
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A.It seems ingredient of section 494 of IPC.
Marriage is voidable not void.
It is difficult to provide legal advice without all the details. However I will advise consult a lawyer and disclose everything to him so that you will get exact proper advice what you are looking for.

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