Let me see if I can rephrase the problem:

• You want to create a single output csv file, based on a set of distinct csv input files.
• For each input file, you are going to run your script once, and this will append data to the common output file
• (Or, for each input file, you will do one iteration of a loop in your script to append to the output file? This seems like a reasonable approach.)
• You need to make sure that the line containing the "csv header" (listing of column names) is written only once, at the very beginning of the output file.

If that's a fair description of the problem, then you could use the following logic (expressed here in pseudocode):

open a given input file
open the output file
read first line of input (this will be the csv header)
look at the size of the output file
if output file size is zero, write the csv header to output
loop through remaining lines of input file
   write lines to output when appropriate
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But if I've drawn the wrong conclusion about what you're trying to do, please clarify.