I ran into this problem about a month or two ago, and don’t remember how I fixed it.
Every time I build my solution which uses T4MVC, I get this error unless I manually delete the file. Also, note that it is “T4MVC1” and not “T4MVC.” This happened the last time as well, where somehow the solution thinks it needs to name the file differently.
I will post the solution whenever I figure out what it is so that it is documented for myself the next time it happens.