Received a 1099-C form for Cancellation of Debt from collection agency. Did not include in tax return, already filed amended return for another mistake.

Yes, it is recommended that you file an amendment and include the 1099-C.  

Although, I would hold on to the document and do nothing right now.  If you do need to amend your return for some other reason, then add the 1099-C. You have until April 18th, 2017 to file the amendment so give yourself some time just in case your receive additional tax documents in the next few weeks.

However, If you do decide to amend your return without the additional wait time then you can follow the directions listed below:

https://ttlc.intuit.com/replies/3318732