I am trying to send multiple trackbacks from my blog to my blog.
However, the max number of trackbacks sent and received by my blog seems to be limited to 1.  Any more than 1, and I get an error message that says something like “trackback already received.”

I tried looking for info in the User Guide and on these forums, and either I am not looking for the right keywords, or the info is buried.  I tried looking in System Administration/System Preferences/Throttling Configuration for an appropriate setting, but trackbacks are not addressed there.  It seems like a throttling issue.

I want to be able to successfully send multiple trackbacks to and from my own blog via the Publish/Trackbacks form.

Is this possible?  If so, what do I have to do?

You can’t have multiple trackbacks from the same URL.  Can you post step by step what you’re doing to make sure we understand exactly what you’re doing?  And are you using these internally or is this from one site to another site that you also own?

Not having multiple trackbacks from the same URL is a problem in my case.  What is the purpose of that limitation?  Wasn’t there a Trackback Throttle function in a previous version of EE?

I am sending/receiving trackbacks internally in this case—sending a trackback from my blog to the same blog at the same domain.  Occasionally I write entries that pertain to more than one archived entry, and I want to add more than one trackback at once.  It seems this is currently impossible.

Here is what I do:
1.  Open control panel
2.  Click Publish
3.  Select an internal blog to publish to.  Let’s call it Blog X.
4.  Create a New Entry.  Paste two or more trackback URLs from Blog X into the Trackback ping area of the New Entry form.
5.  Submit the New Entry.
6.  Get an error message that says something like “trackback already received.”  The first Trackback has gone through, but the others haven’t.

I don’t expect this feature to be changed just because I have a problem with it, but it would be handy for me to be able to trackback my own blog from within the Control Panel as many times as I wanted.

Well, it can be done, but I would keep an eye on potential for spam problems.  If I’m following you right, this should allow it.  There’s one other possible trigger for that error message, and if that’s what is happening, you would need to use a minor hack.

Admin > System Preferences > Security and Session Preferences: set “Deny Duplicate Data” to “No”.

Aaaand Rick just pointed out to check the “max trackbacks per hour” setting in your Weblog Preferences.