It looks like you have some repeated code on the top of the page that would cause the doubling of the quantity.
I've organized the code on the top of the page and attached it to show you more easily.
I didn't remove any code, I just copied and pasted it so that repeated code is closer together.
You can see there are 2 sets of add to cart code for the button "slazenger_1_ATC" on line 10 and line 39 of the file I have attached.
There are 2 sets for "slazenger_2_ATC" on lines 60 and 99.
There are 2 sets for "slazenger_3_ATC" on lines 128 and 158.
There are 2 sets for "slazenger_4_ATC" on lines 187 and 216.
There are 2 sets for "slazenger_5_ATC" on lines 246 and 276.
Each of those buttons would add two items because of the repeated add to cart code on the top of the page. You should be able to delete the repeated code references manually and fix the problem.