Big file uploads and number of records created

I've an application which uploads files between 50 and 100 Megs.

On development everything works as expected. File is uploaded, file is
processed and finally uploaded to AmazonS3.

On production the application works when I upload small files, but
when I upload big files the application somehow creates as many
records as Mongrel instances.

This means that if I've 4 mongrels, the files is generated 4 times and
uploaded 4 times to S3.

As the problem was not being fixed I changed the model to not allow
uploading the file more than once, so I added a
`validate_uniqueness_of :file` ... but Rails bypasses it ... so the
record is created also 4 times even having this constraint.

Does anyone had a similar problem and help me?

I'm running Rails 2.0.2, Mongrel 1.1.3 and Nginx 0.6.25

Thanks in advance,

Francesc