If you have every watched a younger child (or maybe even a young elementary school aged child) count, you may notice he/she having difficulty with double counting. Watch my son counting here:
If you notice, he counts nine bears. However, that is not how many I gave him.
The solution is simple. As long as they know how to count, this should help tremendously.
Have your child move each item as they count. Then they will know when to stop (when they've run out of items to count). You may have to practice with a young child and remind him/her for a while while they remember this new way of counting.