I got a "a non-generated template is already present" error using TcmUploadAssembly
<p>We've been using TcmUploadAssembly to update the C# TBBs in a Tridion 2013 installation, and have recently come across:</p>

<pre><code>Error: Cannot generate template with name Correct Self Closing Elements, 
since a non-generated template is already present.
</code></pre>

<p>In our case "Correct Self Closing Elements" is the name of a C# class. It is contained in an assembly that had previously been uploaded and was working correctly. At this point the C# project compiles properly, the assembly is included as a TBB, and the class is included as a TBB that references the assembly.</p>