I am building an ASP.Net MVC4 web application with Entity Framework 5. I had to use an existing sql server database but also wanted to use Code First, so I followed this tut
I had a similar problem.
The cause for me was that the same migration had been run already in the database but under a different MigrationID. I had 201504302005411_InitialCreate but my migration script was called 201505041911076_InitialCreate.
I fixed the issue by renaming the MigrationID in the database to match the one in my code.
I don't quite know how I ended up in the situation but renaming the MigrationID meant the app knew the script had already been run.