How do I amend a return that I have already amended? I received another 1099-div after I submitted an amended return.

You can amend a return as many times as necessary.

You should wait until the first amendment is processed and accepted by the IRS before sending in the second one to ensure they are done in the proper order. 

You will have to remove/delete form 1040X from your taxes without removing prior changes made. Then you will start the amendment process again and add the new 1099.

You may track status of the first amended one using the below link:

https://www.irs.gov/Filing/Individuals/Amended-Returns-(Form-1040-X)/Wheres-My-Amended-Return-1

 See this TurboTax support FAQ for amending a tax return - https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed

The most reliable way to do this is to let the program delete the 1040X form for you:

1. Make a copy of the .tax20XX file that you used for your 1st amendment.
2. Rename it so it is obvious this copy of the file is for your 2nd amendment.
3. Open TurboTax by double-clicking on the application logo and NOT by double-clicking on the Tax file.
4. On the title screen select "I need to file an amendment".  It will ask you to open a tax file.
5. Open your 2nd amendment tax file.  It will ask you if the IRS accepted your 1st amendment.  Say Yes.
6. When you proceed it will tell you it needs to remove the 1040X from your 1st amendment.  Do it.
7. Follow the procedure for making an amendment.

Thanks to user jeking3 for this solution

https://ttlc.intuit.com/community/after-you-file/discussion/how-do-i-do-a-second-amendment-gt/00/45159