Yes you can repay your HBP early. CRA says:
If you choose to start your repayments earlier, your repayment period will stay the same. Any repayments made before you are required to start your repayments will reduce the amount you have to repay for the first year. If your early repayment(s) are more than the required amount for the first year, the difference will reduce your HBP balance and your remaining repayment amounts over the entire repayment period.
In other words, if you designate your first-60-days contributions on your 2018 return, depending on the amount, you'll either
a) reduce your amount due for repayment next year or
b) reduce the amount sue next year and reduce the overall amount due along with the amount of future payments due.