Why do I get double values?

I don't have the solution to your problem. However I have 2 remarks that might make your life easier.

1 - You take the painful route to generate your Excel file. Some other paths you might consider:
a - If you use it as a raw excel file... generate a simple csv file instead, eg

SELECT a,b,a+b INTO OUTFILE '/tmp/result.txt'
  FIELDS TERMINATED BY ';' OPTIONALLY ENCLOSED BY '"'
  LINES TERMINATED BY '\n'
  FROM test_table;

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(see https://dev.mysql.com/doc/refman/5.7/en/select-into.html) BUT in that case the file will be on the server, not the best solution in most cases
(Note that I used ";" instead of "," as a separator, since I presume your working language in Excel is not English)
b - you can generatethe csv file yourself directly, using the same parameters.

2 - Your code mixes lots of "xml-like" tags with php text. This makes your program highly difficult to undertsand / check / debug.
I would consider placing most of the tags in php $variables, and place these definitions in a separate php file to be included. Your program would then bereduced to 2 dozens of lines .

Good advice offered by fibo.  You would do well to rewrite this script.

Your double values are likely coming from the fact that you have a while() loop based on "$row = $ret2->fetch_array()", but your custom function mysqli_result() is also seeking on the same object, $ret2.  Your row-based loop will also tend to repeat the row closing tag.

Try replacing the nested while for $ret2 with a foreach also (much like $ret3)

while ($row = $ret2->fetch_array()) {
	foreach ($row as $k => $v) {
		echo "<ss:Cell ss:StyleID=\"s24\"><ss:Data ss:Type=\"String\">".$v."</ss:Data></ss:Cell>";
	}
}

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B-)
You're so right Julian!

Thanks Julian. Your insight made my day!

You are welcome.

Thanks Julian! This is something that should probably be added to php.net's notes