Python file.write creating extra carriage return

I'm writing a series of SQL statements to a file using python. The template string looks like:

store_insert = '\tinsert stores (storenum, ...) values (\'%s\', ...)'

I'm writing to the file like so:

for line in source:
    line = line.rstrip()
    fields = line.split('\t')
    script.write(store_insert % tuple(fields))
    script.write(os.linesep)

However, in the resulting output, I see \r\r\n at the end of each line, rather than \r\n as I would expect. Why?