The general category in IRS parlance is known as "Innocent Spouse Relief" and what I have copied below is an extract form the IRS Auditor Manual on what happens in the situation of forgery.
You should share this with husband and focus him on what will happen unless he withdraws the forged return (he may not like what is set forth in Item 7 below if you were to file the Form 8857 alerting the IRS to the issue). Good luck to you.
Section 18.104.22.168.4 (02-26-2013) Forged Signatures
1. When a spouse establishes his or her signature on a joint return was forged and there was no tacit (implied) consent to the return as filed, the joint election is invalid. Again, the relief from joint and several liability provisions do not apply. See IRM 22.214.171.124.13.5.1, Tacit Consent Factors.
2. The individual claiming his or her signature was forged is not jointly or severally liable for liabilities arising from such a return if the signature was indeed forged. However, CCISO should work the forged signature issue, along with the Form 8857, Request for Innocent Spouse Relief.
Note: Line 12 of the Form 8857 asks the RS whether the signature was forged.
3. The account should be adjusted to reflect a married filing separate return and the liability associated with the other spouse only.
4. A married filing separate tax return may need to be secured from the spouse claiming forgery if a return is required for the period, or if the taxpayer may have been entitled to a refund.
Note: There are certain credits not available when spouses file separate returns.
5. A RS who raises the issue of forgery and later determines he or she would owe more tax if he or she filed separately, may choose not to pursue the issue of forgery.
6. In situations where the spouse claiming forgery failed to file despite having a filing requirement, the circumstances surrounding the alleged forgery should be investigated. An interview with the other spouse should be considered when developing the tacit consent issue.
7. Consider referring the individual who forged the signature and any other individual associated with the forgery to the Criminal Investigation Division.
My answer is not intended to be giving legal advice and this topic can be a complex area where the advice of a licensed attorney in your State should be obtained.
Simply write in a brief letter, the same information you've indicated here. This is not your issue, it's your soon-to-be ex husband's to deal with. Just provide the IRS with the information regarding the return you want filed and they can sort the rest out with him.
Evan A. Nielsen is licensed to practice law in California and handles federal tax matters throughout the U.S. The information provided here is for educational purposes only and is not intended as legal advice for a particular matter. This response does not create any attorney-client relationship with the author. For specific advice about your particular situation, please consult an attorney.
Because you live in a community property state (California), you were required to report one-half of the items of income, expenses, credits, and deductions arising from community sources on your separate tax return. In general, the wage income earned after you and your husband began to live separate and apart with no intention of resuming marital relations would be the separate property of each spouse and reportable on only such spouse's tax return. On the other hand, if the income that your husband reported on the unauthorized "joint" return is attributable to the profits from a business that he owns which he started during your marriage (and you had no separate property agreement as to the business), then such profits would be considered community income, one-half of which you each would be required to report on your separate tax returns.
As one of the other attorneys has mentioned, the easiest way to resolve the issue of "your" duplicate 2012 returns would be to write a brief letter to the IRS office listed in the letter that you received, stating that your husband filed the purported joint return for 2012 without your knowledge or consent, and without you signing the tax return. Be sure to send that letter by certified mail, return receipt requested. Within about 14 days after sending that letter, I would follow up with a phone call to the IRS office on the letter sent to you. If the Service refuses to disregard the "joint" return that your husband purportedly filed on your behalf, I would submit a claim for innocent spouse relief under I.R.C. Section 66 (which is available to taxpayers in community property states when a valid joint return has not been filed).