UNIX: cat large number of files - output being doubled - unix

Question

I need to concatenate a large number of text files from a series of directories. All the text files have the same name but some folders do not contain the file and just need to be skipped.
When I use cat ./**/File.txt > newFile.txt I get the following error /bin/cat: Argument list too long.
I tried using the ulimit command a few different ways but that did not work.
I have tried:
find . -name File.txt -exec cat {} \; > newFile.txt
find . -name File.txt -exec cat {} \+ > newFile.txt
find . -type f -name File.txt | xargs cat
and this results in the files being concatenated twice. For example, I have 3 text files named File.txt, each in a different directory, each with a different line of text:
test1
test2
test3
When I do the above commands my newFile.txt looks like:
test1
test2
test3
test1
test2
test3
I can't figure out why this is happening twice. When I use the command cat ./**/File.txt > newFile.txt on my small test set, it works fine and I end up with one file that has:
test1
test2
test3
I also tried
for a in File.txt ; do cat $a >> newFile.txt ; done
but get the message
cat: File.txt: No such file or directory
because some of the directories do not contain this text file, is my guess.
Is there another way to do this, or is there a reason my files are being concatenated twice?

Answer 1

Here's how I would do it
find . -name File.txt -exec cat {} >> output.txt \;
This searches for all occurrences of the file File.txt and appends the cat'ed output of that file to the file output.txt
However, I have tried your find command and it too also works.
find . -name File.txt -exec cat {} \; > newFile.txt
I would suggest that you clear down the output file newFile.txt before you try either your find or my find as follows:
>newFile.txt
This is a handy way to empty a file's contents. (Although this should not matter to you right now emptying a file by redirecting nothing to it can be done even if another process is writing to the file)
Hope this helps.