Finding the number of pairs in an integer Array

Overall it's very readable and fast already. Good job.

I have some suggestions for possible improvements:

• As n must be equal to ar.size, you could drop that parameter from the method and use ar.size in place of n within the method body.
• This method is a pure function except for the side effect of printing the result. Being a "pure function" is often a good thing so you can move the printing of the result to outside the method. Printing something is also quite time-consuming.
• Your method could easily support more than Int, it doesn't have to be restricted by a specific type. You could check for duplicates of any type so we can make this method generic.
• As you are iterating over elements you could use for (e in ar) instead of iterating over the indexes with for (i in 0 until n). This would make it more efficient for data structures that doesn't have a \$O(1)\$ lookup-time, for example LinkedList.
• The method HashSet.add returns false if the value already exists, so you don't need the call to .contains.
• There is a typo in the name enteries, it should be called entries.
• ar could be called input to make it more readable.

After applying all of the above, this is what you would end up with:

fun <T> getNumberOfPairs(input: Array<T>): Int {
    val entries = HashSet<T>()
    var pairs = 0

    for (e in input) {
        if (!entries.add(e)) {
            pairs++
            entries.remove(e)
        }
    }
    return pairs
}