In a bash script, what does `if ((arr[$cksm]++));` do?

I got the below script to find the duplicates from this question

#!/bin/bash
declare -A arr
shopt -s globstar

for file in **; do
[[ -f "$file" ]] || continue

 read cksm _ < <(md5sum "$file")
  if ((arr[$cksm]++)); then 
   echo "rm $file"
  fi
done

I need to know what this line does

if ((arr[$cksm]++));

How does that append to the array? can i try printing the elements stored in that array? I know appending to an array this way

ARRAY+=('foo')

Does the above script use different kind of array?

Please help

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