This is an age old issue in handling dblclick events. Basically there is no way to determine if the first click is a click or the first part of a double-click. Not without adding a noticeable delay.
It comes a bit down to each case. In your example you could simply add 8 on a double-click, as each click adds an additional one. Alternative you could rely on only click with some timers to check time between clicks yourself, and then add either 1 or 10 if a second click comes in within the time given. However, a warning for that would be that each click would have a substantial delay and make your counter seem unresponsive on normal clicks.
The best UX in your case would probably be to stay away from double clicks entirely and rely on ´SHIFT´ + click or even a separate button called "+10".
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