Short Version: yum commands on my Amazon Linux EC2 AMI only work with --noplugins.

Long Version: A couple of days ago, I ran yum update at the behest of the SSH Login MoTD telling me I had updates to install. About midway through the update (specifically while updating the kernel), the update abruptly ended (79 of 138 items completed). The website I host on EC2 got weird for a few minutes, but eventually seemed to stabilize back out (maybe EC2 restarted itself?), and I didn't have further issues (other than MySQL started running out of memory, but I think that's probably unrelated to this).

Today, I went to install gcc-c++ (with yum install gcc-c++). When I did, I got the following message:

Loaded plugins: priorities, security, update-motd, upgrade-helper Config error: Command "updateinfo" already defined

and I get that for any command I can think to run using yum. However, If I throw in the --noplugins flag, then magically it seems to work. To be clear, when I installed a different package a week ago, it worked totally correctly, so the yum update is the only thing I can think of that changed.

I could find nothing on Google with regard to "updateinfo" already defined (with and without quotes). I tried running yum update --noplugins which spit out a message telling me that I should have run yum-complete-transaction instead, but proceeded to try to update something on its own. When that completed, I tried yum-complete-transaction but that gave me a message about the transactions not lining up correctly, so it removed the old transaction (Probably since I should have completed the first transaction before trying to update again, if I had known).

Based on the SF question "Linux EC2 Broken Yum", I've also tried yum clean all --noplugins (fails the same with plugins) which just gives me

Cleaning repos: amzn-main amzn-updates rpmforge
Cleaning up everything

I also tried

package-cleanup --problems

Loaded plugins: priorities, update-motd, upgrade-helper
No Problems Found

and

package-cleanup --dupes
Gives a lot of dupes, so I pasted them here: http://pastebin.com/VVFQEkTT instead of inline.

At this point, I'm not sure what else there even is to check.