Eventually you can deduct the other losses, the reason they don't show up on the assessment is that the IRS does not voluntarily assume you have offsetting losses. The IRS basically never gives you free credit for a deduction, you have to prove it.
If this was an actual error by you where you forgot a W-2G, you would reply by preparing an amended return showing offsetting losses, no tax due, and sending it the office that sent the notice (not the usual amending address) along with a letter of explanation.
However in this case they seem to be double-counting your income. You might want to start by getting a Wage and Income transcript for 2015, see here. <a rel="nofollow" target="_blank" href="
https://www.irs.gov/Individuals/Get-Transcript">https://www.irs.gov/Individuals/Get-Transcript</a>
Maybe the casino or lottery issued a duplicate W-2G. If the IRS has two duplicate W-2Gs in the system, you would need to call the office that sent the notice and talk to someone. If the IRS only has one in their system you still need to contact them because someone duplicated it somehow.
Don't actually send an amended return unless instructed to by the agent you spoke with.
However, DO document your phone call in a followup letter to the office that sent you the notice and that you called. You only have until the deadline to contest this, otherwise it becomes final. So for example, if you call and say, I only won once and you duplicated it, and the agent says "I will research it and get back to you", then write a letter that says, "This letter is a memo of a phone call I placed on _____. I spoke to agent _____ and explained that I only won one time and the W-2G is duplicated in your system. Agent ____ said they would research the issue and get back to me." And send it using tracking and delivery confirmation. That way there can be no doubt that you responded within the stated deadline.
