Stored Procedure throwing TOO_MANY_ROWS at simultaneous access?

Hi,

I have a very simple stored procedure that is part of an application.

Given a part name, it looks up the part_id and returns it. Very simple query, and I've checked the parts table, and the rows are unique. Here's the problem:

Once in a blue moon two people call the stored procedure at the exact same time looking up the same part number. When that happens, the stored procedure throws a TOO_MANY_ROWS exception. What?!

To me, since it is just reading the table, this shouldn't happen. Is this a bug? It is some sort of busy/retry problem I can adjust?

Any clues as to what is going on would be greatly appreciated.

Thanks,