Here's what AdWords considers invalid clicks:
Clicks on ads that Google considers to be illegitimate, such as unintentional clicks or clicks resulting from malicious software.

Clicks tagged as INVALID in ClickGUARD are not the same as AdWords invalid clicks!

There's a number of reasons why clicks could be tagged as INVALID in ClickGUARD and here's a few:

a click from and ad bot (Google ad bot) when you didn't prevent logging these clicks in settings - it's an invalid click that's not shown in AdWords (but it did happen) and you are not charged for it.
a click made to an ad where the IP address was previously blocked - this happens when (likely a bot) saves the ad URL and makes repetitive clicks even though new ads are not showing.
a click with a duplicate GCLID - this is the most common invalid click you'll see and it can happen when someone saves the ad URL and repetitively clicks or clicks on a search ad, does the simple browser "back" action and clicks on it again, repeating the process indefinitely.

How can you know why a click was tagged as INVALID?

Easy, just mouse over the INVALID status and you'll see exactly why the click is invalid.

Are invalid clicks hurting your business?

Not really. But remember - we're talking about clicks tagged as INVALID in ClickGUARD, not invalid clicks reported by AdWords. Those other might hurt you. Fortunately, as you keep using ClickGUARD the number of invalid clicks in AdWords will likely significantly decease.

Why doesn't AdWords prevent clicks with duplicate GCLID?

In reality they will also tag those click as invalid and either not charge you for them or refund you. But they can't really prevent them because of the way URL tracking works. If someone or something saves the ad URL (that starts with ClickGUARD's tracking template) there's nothing to prevent them or it from making repetitive clicks.

Why does ClickGUARD show clicks with duplicate GCLID?

We strongly believe you should be aware of all ad activities and this is why we log these clicks as well.