Why does my T4 template append a number to the file name?

Question

Why do my T4 templates sometimes append a number to the output file and sometimes not? For instance, in one case I might have a template file called Foo.tt and

Answer 1

@itowlson's answer really helped me out, but I discovered a slightly simpler workaround that I thought I'd share.

If you have:

Filename.tt
└── Filename1.cs

Just rename Filename.tt to Filename2.tt:

Filename2.tt
└── Filename2.cs

And back to Filename.tt again:

Filename.tt
└── Filename.cs

Voilà.