How did I get two instances of vim?

Or, more importantly, how do I get rid of the second?

I'm using the vim-session plugin. This morning I've been getting the following error message when I attempt to open a session:

session.vim 1.5: The 'default' session is locked by another Vim instance named 'VIM

1'! Use :OpenSession! to override. 

Didn't understand what it was telling me at first---because I *assumed* I did. Just realized this means I've got two sessions of vim running. Have no idea how I got two. Certainly didn't do it deliberately. 

How do I get rid of the second instance?

Thanks,

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Eric Weir

"You will be needed in the movement when you realize 

that you are not needed in the movement."

- Chris Crass