Odds of Recurrence for Hash Function

Until I am able to process mail in a single file format, I have been
using the hash command to generate file names for storing copies of the
output. The advantage to this is that if there is a problem with the
program that the mail script is feeding, users sometimes panic and
resend a message... repeatedly. By naming the file with a hash of the
content, I eliminate duplicate messages if I need to do a restore.

Since the hash is a finite size and there are an infinite number of
possible content bodies, I'm assuming that there is a risk of two
different messages creating the same hash code. How many messages would
I have to process to have ~10% chance of a duplicate hash on a 32-bit
system? Are there any recommendations for salt values so that
concatenating a pair of hash values would minimize the risk of
overwriting a message with a non-duplicate? 

Chris

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