If the girl dis not get proper divorce from the first marriage, in that case even the husband of the first marriage could not have married another girl. Same is the case in respect of the second marriage also. therefore, both the first and second husbands will never say that they were married to the girl.
That means, you happen to be the first husband (if you have enough proof of legal marriage with the said girl).
But, if you do not have any proof of her further marriage, then you will not be able to say that she is married when you are alive, and without taking proper divorce.
Section 420 has no application in this case.
No case of bigamy is also made out against the girl.