@AsAl36
Start by getting your wage and income transcript from the IRS, that will list the 1099-C as one of your sources of income, if it was issued with your SSN.
https://www.irs.gov/individuals/get-transcript
The instructions for the 1099-C say that if two people are jointly liable for a debt, two 1099-Cs must be issued, unless the debtors are married, living at the same address, and their circumstances have not changed.
For debts of $10,000 or more incurred after 1994 that involve debtors who are jointly and severally liable for the debt, you must report the entire amount of the canceled debt on each debtor's Form 1099-C. Multiple debtors are jointly and severally liable for a debt if there is no clear and convincing evidence to the contrary. If it can be shown that joint and several liability does not exist, a Form 1099-C is required for each debtor for whom you canceled a debt of $600 or more...
If you know or have reason to know that the multiple debtors were husband and wife who were living at the same address when the debt was incurred, and you have no information that these circumstances have changed, you may file only one Form 1099-C.
https://www.irs.gov/instructions/i1099ac
So it is possible that both spouses were issued 1099-C forms for the same debt. You would not be able to verify that without your spouse's cooperation.
The same canceled debt should not be taxable income twice. The problem is reporting this. I can't find any instructions in the usual places on what to do in this situation. Example 3 on page 6 of publication 4681, which explains how to handle canceled debts, clearly shows a post-divorce example where both spouses receive a 1099-C for the full amount, but agree to split the debt based on what it was used for. However, the IRS doesn't say how each individual spouse is supposed to document that the amount reported as canceled debt is different than the amount on the 1099-C.
https://www.irs.gov/forms-pubs/about-publication-4681
Because you have a copy of your ex's tax return showing they declared and paid tax on the income, I think your response at this point should be to write a letter of explanation to the office that sent the notice (the notice should have an address). Include a copy of your ex's tax return showing the income, and include a copy of your divorce agreement showing their agreement to be responsible (you only need to attach a copy of the cover page, the page that talks about the debt, and the signature page, the IRS doesn't need the rest.). In your letter of explanation you can state that since your ex agreed to be responsible, and did in fact pay the tax, and the same income can't be taxed twice, that you don't owe any tax.
Your original problem is caused by not reporting the 1099-C on your return, but the IRS instructions don't explain how to report the 1099-C in the situation where the other ex-spouse has agreed to pay the tax, so I'm not sure what you would have done originally to prevent this.
